Why does the Omnitrix not easily detach from the human?

The Omnitrix latches onto Ben's wrist right in the beginning. I don't think we are told whether it CAN come free if it wants. I'd always imagined that it chose to stay attached in order for the transfers to happen, as if it wanted to create the dramatic events.

Did you see it that way? Omnitrix can only transform the boy wearing it into different sapient alien species, providing the Codon Stream has a DNA sample of the species.