You seem very naive if I'm honest. And very lucky. My relationship fell apart because, I was 18 when I fell pregnant, my baby was unplanned, ex was is immature and struggles with a lot of (then undiagnosed) mental health issues. We split up when I was 23 weeks pregnant due to the stress of our less than ideal circumstances and the lack of family support. We didn't have a clue what we were doing.
DD was born, diagnosed with a life threatening heart defect at 14 days old, had emergency open heart surgery at 17 days old, in the following weeks she wouldn't drink anything so after she was discharged the longest we ever made it out of hospital was 11 days. She then got admitted to hospital at 11 weeks old for a long term NG tube, had a cardiac incident, ended up on the cardiac high dependency unit for a month while they tried to stabilise her and bring her weight up from 7lb to 10lb to minimise the chances of the second surgery killing her, and after a month just had to risk doing the surgery when she weighed around 9lb.
At this point me and DDs dad had tried to repair our relationship, but quite honestly it's pretty difficult to do that when you're both barely adults and one of you is battling pretty major undiagnosed mental health issues and also didn't particularly want the baby in the first place. And that's without the complication of a seriously ill baby, who wasn't even guaranteed life.
So he stuck around from when she was admitted to hospital, to just after her second open heart surgery, and then everything fell apart again and he left because he couldn't cope with the responsibility of a baby who was tube fed every 3 hours, and who had a total of 19 doses of medication across the course of the day.
In all honesty, if DD hadn't have been born, I would have been able to give ex the support he needed, and we would have probably stayed together but I couldn't. And his family certainly wouldn't. As a consequence of the lack of support, ex still struggles massively and cannot be around DD for a variety of reasons that I won't go into here because it's not relevent.
I'm happily married now, and DD is 10 and thriving but... That's why my relationship with her dad fell apart.
There is often real hardship that triggers the decline or end of a relationship.In the congenital heart defect community I've seen relationships fall apart because of the long term impact of life with children much sicker than my DD. I've seen relationships fall apart after a much loved child has passed away after the parents have fought so hard for months or even years to keep their precious child alive.
I'm not saying this because I'm bitter. I went through a horrific period of life, I hit rock bottom a few years ago, and then had therapy to come to terms with everything. It is what it is. DD is alive and thriving, she has an amazing stepdad and I count my blessings every day.
I just think it's important that you know that relationships often end because of real hardship and loss. It's not always just because people let themselves slip into a "roommate" phase.