The capacity for increased intelligence is just our evolutionary "thing". It's just what makes us stand out, like cheetahs being the fastest land animals, or the peregrine falcon being the fastest flying bird. If there's a "reason" for us being intelligent, then there must be a reason for other creatures having their traits too.
If you believe it, question it. If you question it, get an answer. If you have an answer, does that answer satisfy reality? Does it satisfy you? Probably not. For no one else will agree with you, not really.
