When two people become acquainted, then developing friendship depends on the physical attractiveness of each member of the potential pair, their similarity on a variety of characteristics, and the extent to which they demonstrate reciprocity with respect to positive evaluations of one another.

Physical attractiveness
We are often prejudiced about such factors as race, height, weight, clothing, facial features, hair color, and so forth, based on our previous experiences, and are strongly affected by them. Extensive research on this aspect has proved that a physically attractive person is likely to have more friends and form more positive relationships than those who are not. Some aspects of external behavior also play a role in addition to physical appearance; for example, smiling is generally attractive to both males and females.
The Matching Hypothesis - People may prefer the best looking of all possible friends and marriage partners, but not everyone can find such a partner. There is a tendency to select partners similar to oneself in physical attractiveness. Married couples, regardless of age or length of marriage, are similar in attractiveness. Mismatched couples are more likely to break up than couples similar in attractiveness.
Similarity
Have you noticed how we generally tend to like the people whose attitudes are similar to ourselves? This is mostly because our views and opinions on various subjects are ‘validated’ by the other person, who goes to prove that you are correct, and this makes you like the person. Disagreement, on the other hand, suggests that you are wrong and elicits a negative emotion causing you to dislike the person. This may not hold true always, the negative feelings can be eliminated if we know that the dissimilar other is not going to reject us and that he or she is open to discussing alternative points of view.
I like you if you like me
One of the most influential factors affecting your attraction toward a given person is that individual’s evaluation of you. Verbal and nonverbal signs of mutual respect, consideration, interest, wanting to be together, wanting to communicate, affection and liking; strengthen friendships. Once relationships are established, we expect our friends to like us and to evaluate us positively, and to behave kindly towards us.
Falling In Love
It is easy to fall in love but difficult to maintain a relationship. When two people regard one another as ‘prospective’ marriage partners, a relationship can move beyond friendship. The cupid may strike and cause them to “fall in love”.
Passionate love versus companionate love
There may come an instance, during the course of interacting with a member of the opposite sex, when we feel that we are “falling in love” with that person. We then tend to ask ourselves this question, “ Am I in love?” There are at least two quite different kinds of love, and the answer will depend on exactly what we mean by the term.
Passionate love is an intense, sometimes overwhelming emotional state in which a person thinks about a lover constantly, wants to spend as much time as possible with him or her, and is often quite unrealistic in his or her judgments about the person. There are three most important considerations for a person to fall in love passionately-
a) One must be raised in a culture that believes in the concept and teaches it to young people in fiction and in real life depictions.
b) The presence of an appropriate love object.
c) One can easily interpret any emotional arousal to be love. This probably is more of an infatuation than love.
Companionate love: A close and caring friendship.
Companionate love is a deep and lasting friendship that involves various factors such as positive emotions, similarity, reciprocal liking, respect, love and caring about the other person. This kind of an emotional attachment eventually makes a lasting relationship possible. A passionate relationship can also mature into a companionate love, eventually.
In a mature relationship, two individuals treat one another as close friends and value each other’s welfare and happiness.