I don't think it's "love" that is necessary for the sideways to island flashes.
Charlie almost died, triggering a strong emotion that was related to the island. Remember, Charlie died from asphyxia before (Ethan hung him). THAT'S where he saw Claire. Therefore, he was continually connected. I think it's a "once you're in, you're in" thing.
Desmond almost died as well and had a similar underwater moment with Charlie. It wasn't love, it was a strong emotion connecting to another strong emotion.
We don't quite know Daniel's trigger with Charlotte, but he seems to have always had a really strong connection to her.
I don't think it's a near death experience that's necessary. I think it's simply a strong emotion related to something that happened on the island. A feeling of deja vu, but that event DID happen, just not in this life.