- I don't give him as much sex as he wants (I simply don't feel like it)
- I'm depressed/anxious/autistic, which affects our relationship
- I have a crush on someone else
- I feel desire to be with someone of the same sex
- I'm non-heterosexual and non-cisgender, which is something I can't really share with him (he knows I'm bi but knows nothing about my gender), which makes me feel bad

Etc.