A matter like this purely depends on the two parties involved.
Some people are more than capable of interacting with the opposite sex, developing a friendship relationship, and not letting that transition into something emotional and more serious. Others however, may realise the good qualities in said person and thus develop an attraction that wasn't initially present at first.
However, it may also depend on the initial conditions upon which the two met. By that, I mean sometimes the two parties will meet in a purely non-relationship way and decide to keep in that way. Or, you could be one of those people who interact with the opposite sex only when you appear interested in them and want to pursue something more (aside from those you are forced to befriend, such as work, Uni, etc).
Hence, there's a lot to consider. Though it does appear that more often than not, one will want something more whilst the other doesn't. That being said, it's no doubt possible for two to actually be "just friends", which may take a bit more effort to and sometimes require one to forgo any feelings.