Reasons my kid is cranky... there's always something. Hah. There are growth spurts - they want to constantly eat a ton - there are mental leaps, where they are extra fussy and usually once they get through the "fussy" stage they have learned something new or just seem more alert/older, in a few months there are teeth coming in that make them cranky, and then (my personal favorite -lol) sleep regressions. Those are from hell and make you want to die. So be warned. The 4mo sleep regression is coming.