In jiu-jitsu people often look at various training partners as weights.  That guy is like trying to lift 100 pounds, she is like trying to lift 200 pounds, Jay is like trying to lift infinity pounds, and so on.  When I say 'lifting' in terms of jiu-jitsu, I am not necessarily talking about "winning", but rather being able to do something against a particular training partner, achieve some goal (i.e., sweep, pass, submit, or maybe just not allow the training partner to advance past a certain point on the ladder).  Let's say passing the guard of a particular training partner is like lifting 200 pounds, but you can only lift 100 pounds.  You get better, and eventually you can lift those 200 pounds (i.e., pass their guard).  The problem comes because people tend to lock a training partner into a particular weight; that a particular training partner is always like lifting X pounds.  Why's that a problem?  Let's continue with this scenario to highlight why.