Why do Muslims love a few Muslims to the exclusion of others?

A Muslim has rights over his Muslim brother, including: the right of brotherhood and loyalty, and wishing good for him. The degree of a Muslim's love for his Muslim brother varies due to several reasons, primarily: the varying levels of righteousness and piety among Muslims, and the existence of other causes for love such as compatibility between hearts, spiritual affinity, benevolence, or spiritual beauty. Love arises from a harmony between souls, either inherent or incidental. Inherent compatibility is the agreement of characters and the similarity of souls, as pure souls yearn for traits of perfection. As for love for a specific purpose, it quickly fades.