I think I'm on board with Tonberry. I don't like to feel left out or neglected. I like to be invited to get togethers, I like to be asked out on dates or told that I am important to my lovers.
Communication is also extremely important. When I don't text or talk to my partner's girlfriend for a while, I can start getting insecure, imagining that she's mad at me or is excluding me. My boyfriend at times has trouble telling me things he thinks I don't want to hear based on issues we had when we first opened our relationship, so sometimes he'll withhold information about a date or something he'd like to do with someone else out of fear of hurting me, which understandably hurts me way more than hearing it right up front
So yeah, being left out, feeling communication blocks or having information withheld from me or delayed in the telling can cause major insecurity issues with me.