I believe just "friendship" is possible between a guy and a girl but it's usually rare that at least one of them doesn't start to develop deeper feelings. I believe it's easy for one to sometimes wonder if the other is starting to feel something different. Sometimes there's a fine line between love for a friend and LOVE for a friend. If one has stronger feelings for the other then simple things like calling them more often or giving a "friendly" hug can be misinterpreted by the one hoping the friendship progresses into something deeper. And then feelings begin to get crushed. After that, will there be much hope for the friendship to live?
May be the either of both proposes the love first and in most of the (90%) cases, this will lead to quarells between them and along with this there will be some common problems like misunderstanding of their relationship by the society, family members etc.,