When they don't show the same level of responsibility or acceptance you do. When they act on impulse more than well-thought out decisions.
I finally met my ex who I had a long term relationship for a year, in person last summer. It felt like a honey moon we traveled his country (England/Scotland) and I felt comfortable and relaxed with him. Though some things bothered me. He wasn't very accepting and kind of hypocritical. I started to feel like I couldn't express my views about things without him jumping on me. I struggling with my faith, not that I don't believe, I just not concrete on the extent of my belief. Well I mentioned my belief to him and he flipped (he's a very aggressive atheist) He was also more concerned with frivolous things such as video games ( I love me a good video game) instead of making sure he could take care of himself in a fully independent way. When I returned home it started getting worse. I would go to work and the one day off I had at the time I would want to use to talk to him but he continually would go out with his buddies on that night.
When I met someone else who was on the same wave length as me and we became friends my ex got very upset and ended up having a temper tantrum and breaking up with me.
When I look back I'm glad he broke up with me, though I wish I would have seen the real him before I spent 3000 dollars going to the UK. I could have spent it or saved it for an even better trip with my bestfriend instead.