Why did Hannah Baker kill herself in 13 Reasons Why?

Justin ruined Hannah's reputation by taking a picture of her showing her underwear during their first date. This picture was later sent to the whole school by Justin's friends.

Alex created his list which included Hannah. The list ruined Hannah's reputation even more and made others believe she is an easy girl. Alex also abandoned Hannah and dated Jessica.

Tyler ruined Hannah's reputation even more by stalking her, taking pictures of her in her bedroom and then circulating the pictures around the school.

Courtney lied about the picture that included her and Hannah just to protect her own reputation. She told a different story about the picture and stopped talking to Hannah. Hannah felt really bad about losing another friend.

Marcus dated Hannah for one day just because he believed she was easy. When she refused to comply he called her an easy girl and just left during the date.

After Zach and Hannah had a small fight Zach started removing the positive anonymous messages Hannah used to get and so he assisted in ruining her mood.

Ryan took Hannah's personal poem and published it in the school magazine thus causing her lots of embarrassment since the poem was too personal.

Sheri left Hannah alone in the street in the middle of the night because the later insisted on calling the police to tell them about the stop sign Sheri crashed. Later Jeff was killed in an accident because of the same stop sign Sheri destroyed. (See Why didn’t Bryce Walker get the tapes in 13 Reasons Why?)

Hannah whiteness Jessica's rape by accident. She was devastated by Justin's indifference towards the event.

Even though Hannah said in the tape that Clay wasn't one of the reasons she killed herself still Clay's passiveness resulted in more emotional devastation for Hannah. Clay should have told her about his emotions earlier and gave her more support.

Bryce Rapped Hannah. This was one of the most devastating events that happened to Hannah in school.

Mr. Porter didn't give full support. Even though Hannah went to talk to Mr. Porter right before she commits suicide the man wasn't that helpful and he just allowed her to leave.

Hannah was extremely lonely as she didn't have enough loving friends. She lost most of her friend and got betrayed by the rest. The loneliness was more than she could take.

Hannah was bullied by so many people in school. She was trying hard to make friends but most people were responding with bullying and rumors about her.

Hannah's parents were too busy with their own lives that they didn't notice the pain she was going through. Even though they were loving parents they failed to notice any warning sign that Hannah is going through a lot of pain. (See Why 13 Reasons Why TV is a good show?)