[nemzy]

why is it that intramolecular reactions are usually must faster than inter-molecular reactions?

[plu]

According to collision theory, intramolecular reactions should be faster than intermolecular reactions simply because intramolecular reactions involve only one molecule (unimolecular) while intermolecular reactions involve two molecules (bimolecular).  An analogy: If you want to go watch a movie, it would be much faster for you to go watch the movie yourself than to find a friend to watch it with (although going with a friend would be more fun  )

[movies]

I like to think of this problem in terms of entropy or relative concentration.

To undergo an intermolecular reaction you have to overcome the entropic penalty of bringing two molecules together.  However, in an intramolecular reaction, the two reactive sites are already close to one another because they are tethered, so there isn't the same entropic penalty.  (Although there still is an entropy component.)

You can also think of it this way: You know that things tend to react faster at higher concentration because there is a greater chance of the two reactive parts hitting one another that way.  If you think of the concentration of the two reactive ends of a molecule undergoing an intramolecular reactiong, the two ends are always close and therefore have a high local concentration.