I actually started to think about the title partners in crime.Partners being the key word.Why do we choose the partners that we play with.For many finding a solid lefty player to play the left side is the ultimate situation for most.I do not think we would pick our spouses or friends on the south paw basis.There are also only so many of those lefty s to go around.I would say the following reasons are factors

 Manuverability and retrievability

 Aggressiveness at the front wall    

 Superior backhand or forehand

 Ability to play through different court conditions(Glass,concrete,panels,humidity) 

 Endurance and speed

 These are all very important factors that can relatively be gauged,but The most important is compatibility on the court.Some teams ,especially long term established teams,seem to know the right place to be.When mistakes are made they never beat up on their partner.The compatible team can adjust to strategic changes from the other team rapidly.Vic from Classic said it the best once.Watching a good doubles team is like watching a well executed dance.One partner goes in,and one comes out .One always covering the other fluidly. The greatest success of a good doubles team is being able to be a  winner as well as the loser.A great team knows they will win together another day and not every day is a day in the winners circle(unless you are Aimee Ruiz) Jon "Hit Smarter not Harder