It's always easy, when watching a movie or reading a book, to say "yea, so and so and their friend's relationship needed to be developed more,". Hell, I say that all the time. However, at least with myself, I would say it but not be able to pinpoint how exactly that needed to be done.
Developing relationships in between two separate characters can be tricky if you don't have a good grasp on each character's personality as well as the general relationship between the two characters, i.e. "how do they feel about each other?" That's generally a good question to start off with in discovering and building a relationship between your fictional works.
Going back to my previous post about Street Fighter, the relationship between Chung Li and her master is just that, a girl that goes to some guy to learn things, the relationship is, in that respect, too materialistic and way too one-sided for me to feel anything for him when we're lead to believe that he's dead.
Relationship development is tricky because you must know and have developed all your characters so that when they interact with each other, we see a bit of emotion from them, be it joy or sorrow or indifference (yes, indifference is an emotion, or, at least in this case it is because that character should be indifferent for a reason). Your main character should not be the only one that gets something out of a relationship, that's generally not how relationships work... unless they're just that kind of relationship, but even then there's a reason why it exists (and is usually a point of conflict.)