Gaps between the upper two front teeth may appear for the simple reason that there is more room than necessary to accommodate all the teeth. Otherwise, it may be that the muscle attachment (frenum) that is found between the upper lip and the upper dental arch (in the area of the two front teeth) is too large of fibrous.  If so, the front teeth may splay.  A simple surgical procedure may be all that is necessary in this case to relieve the pressure exerted by the frenum.  After that, the gap between the two front teeth may be closed using a bonding procedure that builds up tooth material on both sides of the space.

P.S. A frenectomy is a surgical procedure in which the frenum is cut away from the gum so that its attachment will be further away from the teeth.