As an ENFJ, I have no choice, I connect to everyone, sometimes all at once. It can be harrowing. As to deeply connecting (fusion), I still experience this frequently. More frequently than I would like. I wish I had more control over being involved with people.
I thought this was interesting, since I usually feel the same way -- having a desire to connect with everyone, automatically empathizing, and feeling hurt at some level if I am unable to forge that connection. I never really got to "choose" the desire to do this, it just happens, I want to know everyone on a deeper level, and sometimes I get dragged into people's problems and dilemmas even if it's probably not good for me or I should be keeping my space.
I show the pattern described in the OP in my e-mail communications. If I meet someone I find interesting and who engages me, the first week can be almost obsessive. Usually after a week or two, once the connection is established, everything dies off so it can almost seem like I am disinterested, but I'm not; the connection is now in "maintenance" mode. A few NFs I met with this way seemed hurt by how things dwindled; a few others seem skittish and seem "bait and switch," they'll say a lot of intimate things about themselves, then seem to feel afraid and pull way back for awhile, and it's a pendulum swing between being let inside and being totally held at arms length. Usually with me, it's the slow dwindle, although at any time, that connection can be "called upon" and we can leap into deep things.
Anyway, my online approach is not how I act IRL. I don't know how the ENFs approach it, but I find obsessiveness in the physical realm to be very smothering; the only reason I can allow myself to do it in the writing realm is because I know the other person is always free to back out whenever they need space, so I know I'm not imposing, nor would I pursue someone who shows lack of interest.
Do ENFs get physically intrusive IRL, even when the other person backs off?