Don does love his wife, but he seems to have forgotten the reason he married her in the first place (her looks, sweet personality, chance to start an American dream style family) and just concentrated on her more negative aspects, hence sending her to a shrink.
Now he has lost the other bit on the side (forgotten her name, the one who was very arty), he has in Rachel someone who is at his level intellectually, beautiful, and a little mysterious and intriguing - traits which seems to appeal to Don.