Tony Stark is serious, cocky, and highly self-conscious. He knows that, in terms of technology, he is the best and every time he is proved wrong, he is willing to do whatever it takes to come back on top. He has very little knowledge of social life and has problems interacting with other people (mostly those who are not as rich or as smart as him).
Although he doesn't always show it, Tony really cares about his friends and is deeply concerned about their safety, which is why he always tries to do everything on his own and then ends up getting scolded by either Rhodey, Pepper Potts, or both.
Tony has a tendency to trust the wrong people, who can easily fool him by playing on his emotions. The more tragic their story sounds (especially if topics such as revenge) or family issues are brought up), the more Tony is willing to help, even if his friends oppose to the idea.
It should be noted that Tony also has some anger issues, often snapping or yelling at friends, especially when things go wrong. Tony doesn't cope very well with things not going according to plan (even though he is rarely, if not never, the person who came up with the plan in the first place).