When discussing verse 1, we noted how prophecy is a preferrable or superior gift to be desired, but that desire does not equate to choice, since the Holy Spirit is the one who chooses who will have which spiritual gifts in the church. Paul's point is really to address the motives that one might have in desiring a particular spiritual gift. His message is ultimately that spiritual gifts are supposed to build up the church in love, and that is the motive which should drive our desire for spiritual gifts.
Notice how verse 2 starts with a "for". What's the "for" there for? The word "for" means "because." Verses 2 and 3 are the explanation for what Paul said in verse 1. What did Paul say in verse 1? He said that we are supposed to be especially desiring the gift of prophecy. Verses 2 and 3 explain why we should especially desire prophecy. It's because the one who speaks in a tongue is not speaking to men but the one who prophesies does speak to people. And what is the outcome of the one who speaks to people? Upbuilding, encouragement, and consolation! This is exactly what Paul has already established as being the purpose for the spiritual gifts. In other words, Paul is saying that prophecy is to be especially desired because that person is able to achieve the intended purpose and motivation for spiritual gifts.
It is useful to observe that in verse 2, Paul is introducing an important piece of his topic. In verse 1, he just said to desire spiritual gifts, and of those to especially desire prophecy. In verse 2, he not only explains the benefit of prophecy as a spiritual gift as a reason, but he also provides the downside of tongues as a reason. His phrase, "...for the one who speaks in a tongue..." would almost seem to come out of nowhere, except that we can quickly see that his point is to show the specific difference between the practice of tongues and the practice of prophecy and why this makes prophecy a gift to more earnestly desired. The one who speaks in a tongue is not able to speak to men for their upbuilding, encouragement, and consolation. They are only able to speak "to God" (we are going to discuss what Paul means by this in just a moment). The point of making this observation is to see that Paul has a specific contrast in mind in 1 Corinthians 14. He is comparing prophecy and tongues. We will see as we continue, that Paul acknowledges tongues as a legitimate spiritual gift, but he discuses it as a lesser or inferior gift in terms of its usefulness in a corporate church setting.
I need to open this point by clarifying that "positive" and "negative" here, do not refer to moral positions, such as "good" and "bad." They refer to the matching (positive) side of a contrast and the non-matching (negative) side of a contrast against something. In verse 1, Paul said that we should especially desire prophecy, and in his explanation for why in verses 2 and 3, he supplies a two-part contrast (a contrast to the gift of prophecy) as the reason why.
The first part of the contrast is a negative comparison for why to especially desire prophecy. It speaks of the outcome of tongues, which is different from teh gift he said to especially desire. "...desire...especially that you may prophesy...for one who speaks in a tongue speaks not to men...for no one understands him..."
The second part of the contrast is the positive comparison. It speaks of the outcome of prophecy, which is the same as the gift he said to especially desire. "...desire...especially that you may prophesy...for...the one who prophesies speaks to people for their upbuilding..."
The negative (conflicting) example (tongues) which Paul provides for why we ought to especially desire prophecy is that the one who speaks in a tongue speaks "not to men." Notice how the positive (non-conflicting) example (prophecy) speaks "to people." This is the basis for the contrast and serves to show how the negative and positive ends of the contrast supply an explanation for why we ought to "especially desire" the gift of prophecy. This is directly connected to the idea that we should use spiritual gifts to build up the church, which cannot happen if the gift is not targeted toward people in its practice, for their upbuilding.
There's a second "for" in verse 2. Paul offers the phrase, "for no one understands him." This phrase is an explanation for why tongues is "to God." Paul says, "...one who speaks in a tongue speaks not to men but to God..." Why is this? It's because (for), "...no one understands him." The fact that Paul is providing this explanation here is significant. Paul has not said "tongues are to God because they are addressed to God." He has said that tongues are to God because no one else understands them! Only God can understand the tongues. It's not a bad thing to be understood by God. But it is a bad thing if your spiritual gift cannot be understood by the people who are hearing it!
Let us reiterate so as to hopefully avoid confusion:
Paul says that of the spiritual gifts, we should especially desire prophecy (particularly over tongues) on the basis that tongues cannot be understood by the people who hear them, but prophecy can be understood by the people who hear it.
Consider this example:
Suppose that a guest preacher from another country is visiting your church and shares an entire sermon in his native language, which no one in the church can understand, and does so without a translator to explain what he is preaching. The preacher's sermon in this case is "to God." Because he addressed the sermon to God? No, he addressed it to the church. But it is "to God" because no one in the room understood him except for God! This is the case with speaking in tongues. Paul never said that tongues were "to God" because they were addressed to Him. But he did say that tongues were "to God" because no one else understands them!
We will get into this further later, but I think it's necessary to make the point here to avoid potential confusion and concern. Tongues are a legitimate spiritual gift, and Paul affirms this several times, going so far as to specifically instruct that tongues are not to be forbidden. Specifically in verse 2, Paul affirms that the unknown tongues are "mysteries in the Spirit." We could infer that because of this, the content of whatever is said is "God's will" so to speak. But while the speech itself may be "God's will," this does not mean that each instance of practicing unknown tongues is occurring in a fashion that corresponds with God's will. As a simple example of how this can be so, consider 1 Corinthians 14:28 which says that if there is no one to interpret the tongues of the speaker, then the speaker is to "keep silent." If the speaker chose not to keep silent and to speak his tongues anyway, the implication is not that the content of the tongues was not God's will. It's that the practice of the tongues in that instance was not God's will as we see revealed in verse 28.
Paul does not make the issue about content. He makes the issue about edification, or the lack thereof.
Some people believe that verse 2 is describing prayer language. But Paul did not say anything about prayer or even about who the tongues were specifically addressed to. Paul only says here that the tongues are "to God" as a result of (for) being unintelligible to the people who heard them. The direct implication is that if what was spoken had been intelligible, the tongues would have been spoken to the people to the people hearing them (this is what happens with prophecy). To claim that this verse is describing prayer language is to read that idea into the verse (eisegesis). There is nothing in the verse which says that these tongues were "to God" because they were addressed to God. It only says that they were "to God" because (for) "no one understands him." It is word noting that verses 13-19 do bring in the idea of prayer language, but the mention of prayer language in those verses does not provide a reason to see prayer language in verses 6, 9, or 27. But further, it's also worth noting that when we get to verses 13-19, we will find Paul asserting that interpretation is needed for prayer in tongues as well. Here in verse 2, it is not apparent that prayer language has anything to do with the context without reading it in.