Why can't I love him?

I've tried everything. I can't understand why I can't just love him. It's been 4 years and I am far too comfortable to leave. I have tried everything to love him, but I just feel nothing for him. I don't know if I'll ever leave him or stay. I don't want to ruin a perfect relationship over some lack of feelings. Everything is great. But I feel nothing towards him. Of course I care about him. I also feel that if I ever leave him for someone I have feelings for (not cheat) it will just be a phase/short term romance.