Whoever started the stigma that emotional baggage is selective is wrong. We don't go try to find the baggage - sometimes it is unavoidable and it sticks with us. It may seem like a hassle to stick around when someone seems to have their own, but in reality, you probably have your fair share, too.
Emotional baggage means that a person may have bad memories that stem from virtually anything that could have happened in the past that can change the way they feel about certain ideas. Instead of shaming a person or telling them to get over it, try accepting it and realizing that, to a certain degree, it is normal for them to feel this way.
Accept that your significant other might not be able to go somewhere because all they can remember are all the bad things associated with that place. Accept that they may not be able to be at parties because they've had problems in the past with their partying getting out of hand. Accept that, sometimes, things happen that can change how they think about relationships or friendships.
You may have to make a few sacrifices for them.
I'm not saying you should carry someone's baggage around if it is about an ex they are probably still in love with. Don't feel like you have to stick around if they did something violent and unforgettable to their partner in the past. It is okay to walk away when it reaches a certain level that you know is just too much.
On the contrary, though, don't pack up and leave because they needed reassurance that you need them. Don't give up because they were nervous to let you go alone to a party when, in reality, it's only because they care. Stop running away when someone takes a little bit longer to trust you. Recognize what you're dealing with and what they're dealing with because it usually isn't their fault.
Reassurance of your love might seem annoying because they should just know how much you love them. Other small reminders might seem pointless and tedious to you, but they mean the world to them. Be sure to note that you probably have baggage, too, and you may need reassurance in different kinds of ways— don't make them feel invalid for having their own.
The past is a huge part of what makes a person unique, but not every time can it have a positive impact.
Sure, some people take their negative experiences and use them to make them stronger; I like to think I've done that, too. But you can't turn around every horrible situation. Sometimes, uncontrollable things happen that can't be explained, nor can they be turned into something positive. To deny that is to be naive.
Ultimately, it is your decision if you want to stay by someone's side who has issues with their past... but don't we all? Give them a chance— after all, they are only human.