I personally took measures I wouldn't advise of others in self-experimentation a long time back that allowed me to remove fear as a factor from my mental processes, which lands me in those rare circumstances.
Even without fear however, I still experience vulnerability.
The lesson is much the same as that of the classic trust exercise where you place your trust in someone else and fall backwards, in that it is a great feeling to become vulnerable and be caught, but it is just as potent a negative force when that vulnerability is presented and betrayed.
I have also met people who are perpetually vulnerable, and speak about how life is so joyous, but the beneath that thin veil I can see their pain and torment far more clearly than I care to. They try to deny shame, fear, and questions of self worth completely, but they are completely bound by them beneath that facade.
Vulnerability can be a wonderful thing or a terrible torment, and to me nothing demonstrates that quite as well as the vulnerability that people experience in meaningful romantic relationships.