1- Shared moral and ethical standards and that it totally independent of the social level
2- Similarity of beliefs
3- Matching personality type. Introvert; extrovert etc..
4- Lots of shared interests, naturally not made up
5- Closeness of ethnic background
6- Uniformity of the future dreams
7- Shared degree of personal flexibility and tolerance, even if both are short tempered
8- Closeness of the social level
9- Matching level of education
10- Reasonable difference of age
Do you have a different set of priorities? Do you care about something I didn't mention?