It's not immediately obvious why to me, at least most of the time. So, I'll write it out in hopes of solidifying it a bit. There are a few reasons. One, an open heart lets people in, and lets nuance and connection flourish. Two, an open heart can handle not getting its way, can accept that not each person is not your only chance for connection, whether platonic or romantic. Three, it feels better. It's hard to imagine how this could be the case, as fundamentally much of openness is accepting things you don't like, not getting what you want, and hurting. How could that be better than just... not doing that? Because inevitably, you will not like everything that happens, get all of what you want, or avoid all hurt. You will stress about these less if you accept from the beginning that they will happen.