how to shoot somebody who outdrew ya

I wish I wasn't this cynical. I want to have faith that happiness isn't momentary and that love can last. I feel like love is just a feeling someone has, a feeling like any other and it will fade. The way I see it is married couples take the step to get married to prove the love they have for each other but the love doesn't stay the same way it was from the beginning. It changes. They have children and have to stay in the relationship and then they end up resenting each other for it. They love each other because they have to. I'm afraid to death of that. I'm afraid that the love I have for someone in the future won't stay 100%. I want your devotion from start to finish. Forever. Anything else is not good enough. Is that normal?