cries for no reason

My four year old boy cries for no reason all day. and when I ask him why he is crying he stands there and says um and I and I don't know.  He cries as soon as he walks out of his room in the morning, when I call him to tell him anything like it's time to eat to get your shoes on we need to run errands.  If he is in his room playing he is happy and content and will run out and share something with me and we can interact then but any interaction initiated by anyone with him he will cry. I have tried everything from holding him and saying whats wrong buddy? to asking him if anything hurts to  sending him to bed till he is done, I have tried everything in between and nothing seems to work or help me figure out what the problem is.  He is very capable of expressing himself and does so all the time and interacts with his sisters just fine.  He has no speech impediments and has a good vocabulary for a 4 year old and can cleary express himself when he wants something so I just don't understand this at all and at my wits end.