He isn't. He acts like the older person you know. Providing the all the widsom you could ever need. He is only dangerous because he knows so much, and has the ability to travel back and forth through time as he pleases. The danger comes in the sense where he can make a mistake. Nobody is perfect, not even the Doctor. When he does make a mistake its all, wibly wobly, timey whimey.
Don't get me wrong the Doctor is great. However he isn't safe he knows too much and has too many enemies. He fights for the good so much that his title has changed it's meaning. In "A Good Man Goes to War" River says the Gamma Forests translates "Doctor" as "Mighty Warrior". Also he thought he killed his own people and the Daleks in the Time War. Many other race thought that too so they think he's dangerous.