If you do not know the reason, but just feel, with my boyfriend "that something is not right", then try to find out about the problem. This does not mean that the urgent need to "get into the soul," to bother him, tormenting questions. Maybe he trusts you enough to tell you all that at the first conversation, but maybe you have to have patience. Most importantly, learn to listen. If you are completely self-absorbed, their problems, express their opinion, it would be difficult to understand each other. This means that the focus during a conversation on the opposite side of the words, do not interrupt, do not criticize, do not hurry. Just listen.
When a person is morally difficult, he needs to talk. In this case, sometimes it is not particularly important to anyone's opinion - just need to throw out everything that had accumulated in the shower. If the situation did not seem serious to you, do not tell others about it. So can damage the relationship. Better sympathize and try to give advice. Try examples of situations "over" and talk about what you would do. If the problem is weight and reduce the human experience can only time simply will serve the "vest", hug each other, show maximum understanding. But only at first.
Followed by gradually pull out its slough blues. Offer to go somewhere together, meet with friends, go to the country, to go shopping. In general, do your best to prove the man, life goes on, no matter what, and it is beautiful. Add to the existence of other, as much as possible of positive and positive emotions. Do not let him get depressed and stuck on their problems.