Several studies have found that, on average, there’s some physical similarity between one’s parent and one’s partner. That is, your girlfriend might well look a little bit like your mother. This physical similarity is apparent whether you ask strangers to compare facial photos of partners and parents, or whether you assess things such as parent and partner height, hair or eye colour, ethnicity, or even body hair.
Perhaps it’s an evolutionary thing?
A wonderful study of all known couples in Iceland across a 165-year period found that those with the most grandchildren were related at about the level of third or fourth cousin – no more, no less. So it seems there is some evolutionary advantage to finding traces of parental features attractive.