Why would a girl not actively try in a relationship but would put more effort into living as a roommate to her friend?

So one thing that frustrated me after our breakup was our relationship was often one sided I was driving myself into the ground to make things work in hopes that she seen my dedication and would reciprocate but that never happened. We had 3 months left on our lease and she was going out doing things with her friend in an energized way that would have solved some of the arguments in our relationship. Like I was frustrated why stay in a relationship for 2 years if your not going to give your all she was willing to do more for the friend then me

Why would a girl not actively try in a relationship but would put more effort into living as a roommate to her friend?
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