There are many reasons: 1) Michael was hanging around children too much and Lisa got jealous because she wasn't the center of attention. 2) Michael was going to go on vacation somewhere with some kids and he didn't tell Lisa and when she found out, she a dit he was so selfish and they got into a little fight about it. 3) Lisa thought Michael wasn't ready to be a father yet being a child himself and because of all the crap about the "accusations" that went down so Michael a dit if your not going to give me a child there are plenty of other woman out there that will be willing to give me one (that's where Debbie came into things).
the reasons above are all true and once Michael realised Lisa wasnt going to give him children, that was the worst betrayel in his eyes. She lied to him and that was unforgivable. So Michael put up his walls. He shut her out. Once Michael built his walls there was never any going back.
Although she made him happy for a brief period of time and their marriage, Lida Marie just didn't understand how Michael was really was, nor what made him tick. As stated, par one other members in this club, Lisa Marie was quite upset that she wasn't center of his world she had a fit.