When experiencing great love. That can occur in many circumstances, and having sex can be one of them, all right (depending with whom and how you feel about it)...so can playing music...being in the company of people you love...praying...singing...being inspired in any way whatsoever.

One can also feel very alive in moments of extreme emergency or danger, because one loves to be alive and those situations demand full attention!

It's 100% attention and focus on someone or something that makes a person feel most alive.