Confidence - I hate when people ask if they can tickle me. Of course I'd strongly prefer asking over doing it if it's someone I don't want to play with. But if you have to ask then chances are we're not close enough for me to feel comfortable playing with you. There have been
very few exceptions. Asking if I want to have a session is different. That kind of is required, almost regardless of how well we know eachother - if only because it requires us to get up and go somewhere, so we've got to be on the same page. Unless it's a sneak attack

Few real sessions just happen by accident. Even then, I'd rather be told than asked.
Sense of humor - I love when my lers talk to me, and I love it even more when they make me laugh completely regardless of the tickling. Tickling is social for me almost more than anything else, so I like for us to share similar personalities and a similar sense of humor.
Size/Strength - It's shallow but I'm not gonna lie. I can't feel submissive to somebody who I feel like I could pick up and lazily toss across the room. There have been instances where this is overcome by someone with incredible confidence and/or that extra special, "I'm in charge and we both know it." vibe that I love so dearly.