How can singers do like this?

Well, I guess that's part of what talent is all about. But, you're right. Maybe he did talk to the audience in order to maybe set his mood so that he could sing a more somber song.

yes, i think he remembered something and that makes everything so sentimental, a melancholic event that turn the rock concert in to mellow dramatic scene xD

yes0 a talk could have some effect- the talk could be a time to change what he feels too, to feel the sadness of whatever he is talking about.. being the writer of the song helps the singer to put the right emotion to it.

Songs, have an emotion attached to them if the singer is really good. As soon as he started singing the sad one, the emotions of what he feels well up and make his mood change. The same goes for anyone who might be on a real 'high' and then suddenly receive a bit of bad news. They wouldn't remain that high place, instead they would come to a new level of either sadness, or calmness as their mind reflects on this new information and reacts to it.

I guess you have to have a certain asset to do this. Like the actors who can always hide in a character's body/under a mask and cry out of nowhere for example. It needs an affinity to feel really diverse emotions in a moment. I kind of admire these people. Like, when I feel really happy, and I should be looking serious, I can't often pull it off.