If one grew up in a family of origin where there was little emotional intimacy, then often one subconsciously thinks that it is normal not to have intimacy in romantic relationships and one does not seek it out.

If one was was actually hurt/abused by a close family member, one often fears and actively avoids intimacy.

Thus people from such families of origin often feel violated and/or anxious and/or avoidant when faced with actual intimacy. To them, intimacy often feels frightening and suffocating.