Because we're friends.
My buddy has bought me small gifts over the years. He's always been willing to do me a favor. He has tickets for a concert tomorrow. And I think it was pretty nice of him to offer to take me. This will be the third show he's brought me to. ("... to which he has brought me" for the Grammar Police.) I wouldn't exactly call it a third date, but usually that's when you usially feel comfortable enough to have sex. Our relationship is not that way. Not yet anyway. Tomorrow night I will have a few drinks, we'll see the concert, and afterwards I'm going to suck his cock! I'm gonna give him a nice blowjob In return for all the favors, shows and concerts. I'm going to suck his cock because we're friends.
I'm sure he will enjoy it. I suck off some of my friends, and some suck me.