It totally depends on if our relationship is getting/remaining healthy or if it is not doing well. I have not become bored of anyone, but I have had relationships where we just are not connecting anymore and are unable to fix our problems, and fall out of love.
When I have had a bumpy relationship where we actually are learning from our disagreements and bond tighter after talking things out, well love grows stronger.
I have also experienced being in a relationship with someone that I really like but it just does not grow into love. That came from taking bad advice from friends who told me I should be less picky and date guys I was not attracted to. Horrible advice. I mean if you are picky on a bunch of material things and physical qualities, well that is one thing, but if you just don't have attraction for someone, no spark, no chemistry, you should move on.