We call it many things: intimacy, love, desire, want, need, and yearning. Ultimately, it's about the beautiful, magical stuff that exists between two. What gets in the way? VULNERABILITY: one's ability to be vulnerable in a relationship complicates it. What complicates that?
Here are the usual suspects:
One or both partners' history with closeness; was there much of it, was it healthy, are there wounds that haven't healed?
The presence, or lack of, respect; closeness, without respect is excruciating and makes one want to pull away or leave
One's ongoing negative beliefs; if one or both approach life as one to be survived, and others are to be distrusted, that will affect the level of engagement in the relationship
Whether or not one or both partners accept influence from the other (John Gottman's work)
One must be able to settle one's self down, without relying on their partner to do it
This is an oversimplified list. Please let me know if you have questions or comments!