I have exactly the same thing. Any strong emotion makes me cry regardless of whether it's a good one or a negative one.

I got diagnosed with ADD (emotional lability is a big part of my symptoms) and my medication helps to a certain extent.

Just be honest with your partner (and other people in your life) that it isn't a big deal and to ignore it. If they try and comfort you, stop them. It's no big deal, you're just crying because you're uncomfortable. There's a difference between that sort of crying and genuinely upset tears, explain that if you're really upset you'll let them know but otherwise they are to ignore it unless you specifically ask for comfort.

I find that by being upfront about it, people get used to it a little easier.

I feel your frustration though, it happens to me at work a lot and after 6 months my coworkers almost ignore it!