Mike was already used to having Steve around ever since the 'upside down ' happened. Steve, being 4 years older than them, acted as some kind of babysitter amongst Mike's group of friends.

However, one day Steve brought you along, you a very pretty girl, and Mike was quick to grow attracted to you from the first moment.

Maybe that was the reason he noded his head enthusiastically when he heard you joking to Steve about 'keeping him' "yes, keep me!" Mike said, clinging to your arm.