1 His story impresses some people
Justin Bieber is extremely young and successful, which is the kind of story that can impress a large number of people. Some fans admire the fact that he became successful, famous and rich at such a young age.
2 He believes in himself
Justin Bieber was let down before but he kept moving and believed in himself. Some people admire the fact that Bieber became famous in spite of the difficulties he faced.
3 He created a cult
The people who like Justin Bieber call themselves 'Beliebers', a word that means 'believers in Bieber'. Human beings like to belong to social groups, and because some people like Justin Bieber, they found it very interesting to belong to the Beliebers' group.
4 Some people consider him very handsome
Some people believe that Justin Bieber is very handsome. When adding physical attractiveness to the success package, it makes Justin Bieber much more attractive to his loving fans.
5 Finding him excuses
People usually collect evidence to support their current beliefs and not the opposite. Bieber's fans go lengths to justify any mistakes he makes just because they love him. A typical Bieber fan will have so many excuses ready for the mistakes Justin Bieber made.
6 Because he has many haters
When a celebrity has a large number of haters, he becomes a hero in the eyes of the other party. Some people believe that Justin Bieber doesn't deserve that hate and so they take the opposite road and like him.
7 He is an idol for many young teens
Some young teens see Justin Bieber as an idol because he is one of the very rare celebrities who are close to their age. While a 16 year old teen might hardly relate to a 50 year old singer, s/he can easily relate to Bieber who is very young.
8 Gives hope to teens
So many teens see Justin Bieber as a source of hope. Each teen has his own dreams, and Justin Bieber is a person who achieved his own dreams as a teen. When some teens see Justin Beiber on stage they remember that they can also achieve their dreams just like he did. (See What does it feel like to achieve your dreams?)
9 Females hardly feel jealous of him
Many of Bieber's fans are females. Because females usually don't feel jealous of Justin Bieber as much as males do, they don't find enough reasons to hate him. This is why many of Justin Bieber's fans are females.
10 Some people say he responds to fans on Twitter
Some people say that Justin Bieber sometimes responds to his fans on twitter. The fact that some people see him as an idol, combined with the possibility of replying to them can make him much more popular among people who like him. (See Why do Celebrities like Twitter?)