Why does a convergent boundary occur?
The divergent boundaries produce new crust. The new crust does not increase the size of the earth, The new crust must be destroyed or buckled at some place.
The mid ocean ridge in the Atlantic is expanding westward. The mid ocean ridge in the Pacific is expanding eastward. The expanding crusts moving in opposite directions must meet. When the two expanding plates of crust meet a convergent boundary is formed.
When one plate is an oceanic crust made mainly of shallow dense basalt meets another plate that is a thick less dense continental crust, the ocean crust is pushed and pulled down. The convergent boundary of a an ocean crust and a continental crust forms a subduction zone,
When both plates are continental crust both crusts are buckled upwards forming mountains. This is another form of a convergent boundary.