Why do mosquito bites merely itch while bee stings are painful?

A mosquito inserts poison and a bee leaves half its body behind with a stinger attached to the poison.  The bee loses a lot more than the sharp pain it gives.  Unless you are allergic to bees.  Than it can be life-threatening for both of you. 

Mosquitoes inject a pain killer substance but you may feel the initial 'sting' as he pokes you. After a while, the 'pain killer' wears off and it itches. 

Well, i just know that bees/wasps are important creatures and they want bigger ones to take them seriously and not mess with them !

Mosquitos don't have venom.  Bees and wasps do.