You want players that complement each others playing types. You wouldn't want to put an enforcer with a playmaker.
The best pairing usually consist of a combination of the strengths and weaknesses of players.
Most top line pairings consist of a playmaker, with a sniper and two way forward. Typically the playmaker sniper combo goes well and you can even have two snipers to one playmaker.
Two way forwards can almost go with any pairing. The grinders work well with enforcers.
Think about it like you were actually building a team in real life you put the scorers with the passers and the bruisers with the gritty checking forwards.
