Factors are personal characteristics and evolutionary explanations.

Explanations connecting to Interpersonal Attraction.
Personal uniqueness
It has been found that physical good looks is probably the most vital factor right at the beginning of the relationship, as we do not have anything else on which to base our judgement of the person in question. If a person is physically gorgeous, we automatically expect them to have other positive attributes. This is called the halo effect, and explains why we are more likely to be attracted to physically attractive people. This has been shown in a study where photographs were attached to students essays when the teachers were marking them. It was seen that the more attractive students did indeed score much higher on the essays than the unattractive ones.
Though it may seem that everyone would try to get the most attractive partner possible, this may not be the case. Walster proposed something called the matching theory, and suggested that we pick a partner based on our own level of attractiveness.
Although personality is not the most important thing at the beginning of a relationship, it is one of the most important factors after the initial judgement has taken place. Different countries value different personalities as being attractive, for instance UK and US cultures value independence and extroversion, whereas eastern cultures prefer introversion.

As well as this, the traits that we see attractive at the beginning of a relationship, such as predictability, may become boring late on in the relationship, and may cause issues within the couple.

Evolutionary Explanations

It has been found that women seek partners who can agree them economic constancy, because of the high level of physical investment they put into having children. Because they spend nine months carrying the child, they want to make sure that there is money available to care for the child when it is born.

Men on the other hand have an almost endless reproductive capability, so their level of initial parental investment is low. Because of this, they will search for women who are attractive and will be able to give them healthy, ‘good quality’ offspring. Traits which could show good reproductive qualities are youthfulness in general, and also symmetry of facial features.

This has been shown in newspaper ads, where men would search for a youthful mate 42% of the time, in comparison to the 25% of females who searched for this. Also, 44% of men search for a physically attractive partner, in comparison to the 22% of females.