A lot of it is history; have you more or less always been like that? have you had a growing up that would make learning to handle strong emotions difficult (could write book explaining that more in depth, so I'll leave it there)? Do you have realtives who also have a hard time managing their emotions? In addition, people with BPD often have a very unstable sense of who they are that runs quite deeply, and they tend to take on what most would think to be basic personality characteristics (like sexual orienntation, extroversion/introversion, etc) based on who they are with at the moment. They often have a very pathological sensitivity to abandonment, and have a tendency to see people and issues in a very black and white manner; people are either all good or all bad, and a person can easily go from being one to the other depending on what is happening. The DSM has a general checklist, but there is a good deal more to the disorder than that,as there usually is with everything that has to do with people.
Bipolars don't tend to have the identity issues and the abandonment issues, if they come up, tend to be restricted to depressive and mixed episodes. They also don't tend the have the black and white thinking except at the extremes of the swings. Generally, strong emotions can be a problem, because out stress systems tend to get "reset" making our adrenaline systems overreact, making us a bit more labie emotionally even when our moods are "normal", so the perfectly calm bipolar in a normal phase is not all that common. Yep, professionals can get us mixed up, but these are the biggees when it cames to the differences! HUGGGGSSSSSSS!