depends on the basis of the relationship. for instance if one is placid and withdrawn and the other is needy and always has friends and other guys around them and she is flirty etc. then she will probably move on to someone else, evetually and will find the relationship restless and difficult.
if you have some connection that makes sense then the relationship will last longer. but i have found that females can move on quickly if they feel that can find someone better. it is very common these days, which probably says more about me then them.
i have learnt that if in fact you are just average at things the more likely it is that a relationship won't work. even dating sites sort of play up to this concept, that you can choose people from a menu and try people out, much like dating.
that isn't to say that there isn't the right partner out there.
so wrapping your head around rejection can be the hardest part, especially when you see couples that stay together for long periods. but then you look at those couples and most of the time they are both a well adjusted match that are normal, above normal, successful, above successful, they know they match up, if that makes sense.
you have to have average to good self esteem if you are going to be in a relationship, particularly for males. having low self esteem just doesn't work, unless you make up for it in appearance or if you have something about your personality that people naturally draw to.
i can certainly see how people are in relationships and how some people aren't. some people have genetic advantages or personality advantages.
i usually just assume as protocol that even if i become friends with a female that she most likely has interests in other people, someone better looking or more interesting, every single time, i also assume that for everywhere i go no matter who i talk to. that's just how i see it. most of the time i am right about it. once you become friends you find out that they do like these other (more confident) guys
so i just don't think about it anymore. what's the point? right? it isn't helpful to think about it, it won't change anything, much like worrying doesn't help. sort of like a crush is useless if you don't actually try and talk to that person.