Because your marriage was real life and life with her is still a fantasy. Because he got a rush of brain chemicals and he's drunk on them. They wear off. Because he's a selfish jerk who is only thinking about himself.
See, it has nothing to do with you. And certainly nothing to do with you being "less than" someone else. Don't even go there. You're way better than that.
M60 H52 D20 M14 yrs OW-old gf from 1986 bomb-5/18/08 H filed for D-9/10/08 D final 4/24/09 xH remarried (not OW) 2012