Relationships are work... and if you're not willing to put in the work then the relationship can't last.
I think for me, I'll question myself to see if I'm doing enough work. Have I shown enough affection so that the other person is reassured that the relationship is important to me? Have I done enough housework so my partner has an easy time and I'm not adding to their stress? Have I replied to messages, organized things, etc?
I don't worry too much about what people have done for me. I don't even know why, I just don't think about it.
Sometimes I think 'love' is a bit of a cop out, especially if someone says something like "sure they're awful to me sometimes, but I love them" - I always think that the person saying that doesn't value themselves enough to end a harmful relationship.
I think you're absolutely right that the are different levels of effort for different times, sometimes life is a lot and we need to support or depend on our partners more than usual.