What encourages Bitcoin miners to relay blocks? Lets say I'm mining bitcoins, and I've just received a new block that someone else has mined. What encourages me to send that block to anyone else. 

I can understand how the original miner wants to send the block around the network (so their block becomes part of the chain) but what incentive do other nodes have to send it around? Wouldn't it be better to not to spread the block, but then build your own block on top of it, in which case you'll then be two blocks ahead and more likely to be added to the chain?

Of course, if everyone does this the result would be terrible, but what incentive is there to encourage people (other than the original miners) to spread blocks besides altruism?
As long as the block reward is much higher than the transaction fees, this is totally not an issue: mining isn't an incremental process, it's a bruteforce probabilistic one.

I.e. in a single moment your chance of solving a block isn't higher if you have been mining that same block for some time: the chance is always the same.

Hence, once a block is relayed to you, you just add it to the blockchain and start immediately to search for a new block on top of that one.

Since the risk of your block being orphaned (currently you'd lose 25BTC ~= 3750€) is many orders of magnitude higher than the cost of relaying it (I'd estimate it a bunch of satoshis), it is always more convenient to relay it.