Proximity: The closer two persons live, or work together, or share the same hobbies and the more they interact with each other, the more familiar they become with each other. They are more likely to become friends or lovers.
Similarity: The more similar two people are, especially in terms of attitudes and interests, the more like they are to form a relationship.
Physical attractiveness: People of similar attractiveness tends to pair off, however, women will tend to choose partners with compatible looks. Men however, are less concerned with their own looks when deciding who to date.
Reciprocal liking: We like those people who like us and dislike those people who dislike us.
Competence: We like people who are reasonably competent and socially skilled but are not "too perfect".