Why do men act like the woman is pressuring him to change back into "someone he's not" only AFTER she is invested into his promises?

Why do men sign up for relationships and then revoke the behavior that initially won the woman over. From example, I had a man who persued me every day, who claimed to love me until I believed it. He introduced me to his family and friends, he bought me gifts, and he spent time with me very often. Once I was finally in love and invested, he changed. Him and many other men made excuses that change was natural, but he didn't just slack, he became the total opposite of the man I fell in love with. He stopped spending time with me and only wanted sex, and he was mean at times. I had never done anything to him but I still questioned myself because his change seemed too drastic to be normal. I gave in and accepted lies when I knew he was being dishonest, and he made me feel as though I was crazy whenever I would challeneged his lies. If he was caught redhanded he would say "your not my girl" and he seemed to completely disregard our past and his promises, as if neither had ever happened. He reminded me that he loved me and showed me sides of him that I thought were special by crying to me and telling me secrets. But for a long time he seemed angry at me and at my expectations of him, as if I were asking for more than he bargained for. When I accepted less, he treated me as if my lowered standards were a sign that I WANTED this. Finally, I wanted out. He reeled me back in many times, but him "realising his mistakes" didn't last. He would act as if I was his whole world when we were together, but when we we're apart he ignored me for weeks. I could never trust the same again after I accepted that my time had been wasted. The fact that my situation is COMMON just made me wonder why do guys do this? Why not leave us alone? It makes it hard to believe that men can be good people when they clearly tell you they have intentions to love you and then later treat your love as a pester to them and their motives to be single. ESPECIALLY when you're an all around good woman.
Why do men act like the woman is pressuring him to change back into "someone he's not" only AFTER she is invested into his promises?
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