What really are the reasons for these animals/insects?

Yes, their purpose is to live and reproduce. They also play critical roles in their respective ecological niches. Their stings and venom were evolved primarily to capture their prey. Though they do serve as defense mechanisms, they didn't evolve and survive for millions of years simply to inconvenience you while you play in the outdoors. They serve as predators in their environments. Everything has a purpose and everything fits into its own little corner of the world.