you probably just haven't met the right person. when the excited feelings return for a little while that could be you convincing yourself to keep trying.
some people are 'all or nothing' type of people. that can be difficult when day to day things interrupt and then feelings fade fast. I say try not to think about things too much, and if you continue to feel this way, well you just need to meet someone who is the same as you.
when i was younger i found that i was an 'all or nothing' type of person and when my attention was distracted i got disinterested. as time goes by things change and you'll find that your attention to small details don't matter as much.
there is nothing unnatural about feeling like this. take it easy on yourself. try to relax about relationships don't try to be so perfect.
Work like you don't need the money. Love like you've never been hurt. Dance like nobody's watching