Donald Hebb's postulate only applies when two neurons are already connected. It seems you are asking more specifically, 'when two neurons are not already connected and they want to connect, how and why?' Correct?
In this case, we do not know. StrangeLoop mentions it is due to the location of the neurons and the spreading of activation. Yes these might be the case but evidence is extremely limited.
It may be the case that correlated activation of two neurons instigates the formation of a synapse between them. It may also be the case that synapses form completely randomly and plasticity takes advantage of any new synapses (don't forget anti-hebbian). It may also be a combination of both. We do not know.
We further do not know concretely what happens in the reverse; for example, what causes a synapse to be eliminated, is it activity driven or a stochastic process?
Some recent evidence showed that spine generation and elimination are related to 'learning' and 'unlearning'. This suggests that activity driven plasticity can form/eliminate synapses, but my previous reference shows strong evidence that formation/elimination are stochastic -- hence we do not know!
Another thing which is not clear from your question, as James mentions, is at what age you are considering. Early development of synapses is more well understood and it may be the case that during early development a group of mechanisms determine why/where/how a synapse forms/eliminates and during adult another set of mechanisms are in charge.
For example, in the early post-natal visual system, it is found that cells have responses with similar properties as adult; however, later on in development visual experience is required to maintain these properties. Furthermore, these changes must occur within a critical period for activity-dependent plasticity.