Why would Professor X give his younger self foreknowledge of his entire life psychically? Well for one, to avoid many personal mistakes, but more importantly to make strides for a better tomorrow. Professor X isn't Doc Brown, and therefore has less qualms about screwing with the space/time continuum. The entire affair of Days of Future Past is about traveling back and forwards through time in order to avert great tragedy and terrible circumstances, so it wouldn't be far fetched to assume that the older X would want to arm his younger self with some choice knowledge about how to avoid certain pitfalls, where to find certain allies, and possibly how to refine Cerebro. Of course, that knowledge is only useful if that future actually comes to pass...