How come when you fall in love with someone but in the end you are never satisfied?

There are many relationships in the word..and this question has been stuck on my head for such a long time and i need some answers PLEAZE!!! It's like you are happy being with someone and no one is stopping these feelings for you where it's something you want to make it last,BUT something in the end you are never satisfied.Why is that? What's wrong being happy with one person? I know there are like gazillion hot chicks and guys out there but i know a lot of people gone through a phase but i just truly need an answer...and please dont' tell me it's part of growing up.How come when you fall in love with someone but in the end you are never satisfied?
sometimes its the ppl





everyone is different .. sometimes ppl are satisfied .. and continue there life .. witht he one they love


and some get into fights .. and


and some end up cheating cuz wut they hav is not enough


and some end up cheating cuz there horny .





but it just all depends on the person ..