The moment you've said everything on your mind, suddenly, you've become uninteresting. It's because when we think we've got something figured out, we'll be itching to find the next thing to figure out. When you get handed something on a plate, it's a lot less satisfying than digging it out on your own. And even when you do get it, it's not like you're going to sit around. It's not like you're ever going to be satisfied with what you have.
I don't necessarily mean be mysterious. It's just, be interesting. If I can tell what you're thinking, or you're telling me what you're thinking, you've done all the work for me. It's no fun. I'll move on to the next thing that I can work on.
When you're working on something, you need signs of progress. Don't you love finding out something new about someone after knowing them for a year already? Or when they change their mind about something? Or when they comfort you with a familiar subtle behavior that you figured out on your own?
The thing is, this interestingness, it can go on forever. Aren't we always learning, always changing? If you can keep it up, to become a better, cooler version of yourself every day, then you're always going to be awesome.
And besides, meaningful conversations are born from the thoughts and experiences of two or more people, and often in the moment. You can have the most amazing conversation in the world and not have someone know a thing about you.
Sure, say what's on your mind. Just not all of it. Just enough for the situation. Let them try to understand you. Drop hints. Not just through words, but actions. Facial expressions. Posture. Work ethic. Ask questions. Listen more than talk. Smile more than not. Work more than not. Laugh and joke. Then be serious. Keep 'em guessing. They'll want to hang around to see what's coming next.