it comes with practice and positive responses, feedback, and social support. but most of us, the confusion, the fickleness, the uncertainty, the changing perspectives, the moodiness, the neuroticism, it gets in the way and we rarely deliver a clear (read: uniform and linear) message. people who get us can really fuckin juggle.
the rest is up to the fates. we can either become infatuated with someone right away, or we can grow into our relationships and eventually realize we have stuck around a long time for many good reasons. for being moderately wise creatures, we are often very very poor at recognizing what is going to happen with our own feelings in the future. we imagine the possibilities, but that far down the line, it's really all a guessing game. those kind of holistic judgments are attached to nothing solid, real, or actual. Ti and Te people, when it comes to this, are like a billion times smarter than us.