They're not looking for life on pluto. Its just mainly that its the only planet (well.. of the orignal 9 anyway) that has yet to be visited by any spacecraft or has really been looked at at all in any detail. The best images we have are rather crappy and have no detail at all.
It wasn't until last year that they found 2 new moons around pluto. We really don't know much of anything about the planet except that its really part of a new class of planet that we're starting to discover. There have recently been a lot of discoveries of new planets of the pluto type in our solar system. Pluto just happens to be one of the closest and biggest (though it isnt the biggest. "Planet" "Xena" is bigger than pluto, for example... bet most people didn't even know we had more than 9 planets in our solar system.. Xena is only the provisional name of that 10th planet [it even has a moon named gabrielle
])
Anyway these planets are of a type that is basically a building block for planets. Most, if not all, of the planets in our solar system started out mostly or exactly like pluto is. So in order for us to understand our own planet, the origins of the planets in our solar system.. and other reasons.. we're sending that craft to pluto to give us a better understanding of these things.