As a teacher, it is absolutely essential that you do not play favorites, no matter how likable or friendly a student may be. Other students can tell very easily when a teacher does this, and it can destroy the trust and respect that a teacher needs to succeed.
This is especially true when it comes to grades. If you teach a class that is subjective like English, then it might be a good idea to have a grading system where you can't see the students' names. Do whatever you can to not let your grading be swayed by your opinion of individual students. Each one deserves a fair chance.
There is nothing wrong with getting to know your students, laughing with them, etc. Just make sure that you don't make it look like any of them are getting special treatment from you.