There is a definite difference between loving someone and being in love, in a committed relationship where you are in love with someone. A passing fancy, crush, feelings or love is simply not the same. In a committed relationship there is (or should be) a foundation of trust, security, and respect. You are entrusting someone to come to you about problems in the relationship, you feel secure in your position, and you respect their feelings, questions, concerns and general being. In that type of relationship, a celebrity crush or joking about how hot a co-worker is...it's trivial. You're happy and assured the relationship is stable and there are no fears or doubts.
To tell someone that their position is threatened shatters the security. Comparing their love to someone else's smacks of disrespect for their efforts and everything that relationship stood for. The trust is simply gone, as you have now bypassed talking about a possibly dying relationship and any problems. You've simply decided the person isn't worth it, and you'd rather cut things in a "This person is better than you, you are no longer good enough" type of way. That is selfish and cowardly. To question it, "Maybe I love you more, I think, possibly you might be good enough" is worse. You aren't even considering the other person at that point. You haven't cut the relationship because your position has changed with your partner. It's because you've allowed someone else to come in and get in the way. You've abandoned respect, security, and trust for that person. It's now only about YOUR feelings and position. Selfish.
In no way was I trying to say the feelings can't be there. Just that a commitment is more than just feelings. And you commit, or you don't. I find little wiggle room there.