Why can't we be friends

Eevee is just a few months younger than Lucas.  I was excited for Lucas to have time to spend with his cousin, as Eevee is at Grandma's house a lot.  Within moments of there first interaction it quickly became clear, that this was not going to be the case.  They did not get along at all!  They were constantly on each other, at one point I caught Eevee holding Lucas by the collar slamming him up against the kitchen cabinets.  This isn't to say that Lucas didn't antagonize Eevee as well.  We were, after all, in Eevee's territory, and she saw Lucas as an invader.  Not to say that there weren't a few small moments of peace.  Toward the end of our visit, when we were getting ready to head to Craig's parent's house.  Lucas looked at Craig and said, "daddy no more, no more."  Craig's reply was, "no more what Lucas, no more grapes, no more drink?"  Lucas, "no more Eevee, no more."