team balance

for a match in normal game, what is the criteria for choosing both teams? for me, i have tried it in the past few months. when having consecutive wins, then there will be somebody trolling our team and losing for a few days. some will have gold or platinum rank but bronze performance. i am not finding someone to carry me, i am just finding some normal teammates. is it harsh? can riots fix this problem? i dun want to be trolled by somebody anymore ok?