We all now the basic reason on why Juri broke down- but let me look a little father. What could possibly have happened to Juri to make her reject herself and her friends.
1> The death of her Mother: No one ever can forget something so heart wrenching. Juri was unable to really understand and it was especially bad because she has memories of her dead. Though it seems that she had time to heal she never did. Instead she suppressed the feelings so she could deal with life.
2> Living with her father alone. Although he loves her he is something I would call a stereotypical Asian father. Cold, hardworking, definitely not a mother like figure. He is unable to nurture her like a child needs or openly express his feelings. So f course that is going to have serious repercussions on a child.
3>The remarriage: Kids often can feel bitter if their parents remarry. Juri is one of them. She had felt as if her stepmother was trying to replace her mom, and maybe even herself. Juri knows her intentions are only good but there is always a hidden layer of resentful feelings.
4>Not being a tamer: Juri has shown of self-worthlessness or boredom with her life before, "Nothing special like that ever happens to me." Becoming a tamer is her way of proving to herself she is worth something and able to be more then a face in the crowd. So when she wasn't she came to terms that she wasn't 'special enough'.
5>Leaving for the digital world: Her father had never been a close man. Juri had seemed to want to have as little contact with him when telling hi, "I just had Leomon talk to him." I think it must have ended ugly with the father telling her off.
6>Capture of Juri: Juri had been captured in one of the episodes. Before she had claimed she was not as strong as she wanted to be, and when she tried to help she had just made him stronger. This just feeds into her self esteem issue.
7>Death of Leomon: The most 'duh' answer here. This is her second death of someone close to her by someone. Not only that but she also blames this on herself for not being strong enough.
8>Biomerge: I know everyone right now is looking at the screen funny thinking 'huh?'. But really how great must you feel is not only your digimon is dead but now everyone else can merge with them?
9>Juri's torture: Soon after all of this, before she can even thin about getting things in her life straitened out she is captured by the D-Reaper which takes advantage of her sadness. She is then forced to relive her pain repeatedly in sort of a hellish nightmare.
10>Impmon's sacrifice: So she just wakes up from her nightmare and all of a sudden the face of her digimon's killer is reaching out for her. He is, of course, changed but Juri wasn't there for the redemption. Too add insult to injury- just when she was about to reach for him he uses 'Fist of the Beast King' reminding her of Leomon's death all over again. Then he falls away almost deleted so she blames herself for it. Even though he killed Leomon she doesn't want anyone to die.
11>Realiziation: Juri realizes that it was her pain that fueled the D-Reaper and she is powerless to stop herself and it. She then falls deeper in depression when she realizes that she is the reason her friends are almost dying and watches as the D-Reaper harms Calumon.
So you see Juri has quite a lot to recover from- more than most of us realize. Which is why I admire her- she had to change and force herself out of her depression that she wasn't given time to heal to help her friends. It is said later that she is no longer suffering. That is cool.
