Are they sandbagging you? Friendlies are some of the best places to practice/experiment and good players are usually trying to work on something specific whenever they get the chance. They could be trying stuff out on you because you react better then a CPU, but it isn't difficult to try what they want. Likewise a lot of good players will do what KirbyKaze does to try and drill the basics of spacing and mind games into new players heads(Most new players are tech heads and will improve on that on their own time).
Lastly, I admit, for a long part of my career I had a poor opinion of sandbagging and even today I look down upon it. However, when you reach a certain level of mastery, playing people a league or 2 below you is boring. It's not challenging, and you stop having fun with the game, which sucks. Some people enjoy teaching and fight the boredom with that. Other's bring out their secondary characters and refine dull edges with them in an attempt to get something out of the session for themselves. Other's still will do goofy reckless **** while trying to amuse themselves.
Bottom line, it's definitely a blow to your pride when someone is sandbagging you, but at the end of the day we play this game, compete at this game, obsess over this game, because it is fun. Do you really want your friends to not have fun for the sake of your pride?