I happens. After 14 years, people go from strangers to friends, or at least very good acquaintances. It's old- came for the game, stayed for the people. My gaming group started that way 17 years ago. We formed to play games. Gradually we started to hang out outside of games. These days, d&d is excuse to get together once a week and hang out. Social aspect comes first, gaming comes second. There are sessions when we don't even play, we just talk. But we, as a group, had talk about what we want out of gaming, and for all of us, hanging out and seeing each other comes first, playing game comes second. We aren't gaming group. We are friend group that also plays d&d.
Don't burn bridges, specially if you like them as a people ( and 14 years is damn long time to hang around people you don't like). Instead of treating it as a game night, treat it as hangout time. Be honest and open about it with DM. Say to him that you don't enjoy that focus has switched from gaming to socializing and aren't into committing your time regularly to that kind of game night, but, if you like to hang out with them, you would be ok for occasional session.