Pros and cons of installing a roof on top of another or stripping the old roof off first?

If your present roof doesn't leak putting new shingles on top of the old won't "develop new leaks later on." Your roof will easily handle two layers of shingles but you will find if you try to hire somebody they will almost always want to remove the old shingles and start fresh. If there are any old shingles curling or deformed your new roof will not have a smooth appearance but if your present roof is in pretty good shape and you can smooth out any curling shingles by nailing them down there shouldn't be any problem adding a second layer. It is much less work now to not remove the old shingles, but it will be harder when you reroof in 15 years or so to remove two layers rather than one.