The first reason is obvious: trust. One of the nerve-racking challenges as a parent of a teen is to tolerate their moving away from us, and trusting they will tell us when they need our help and support. It's vital that our kids feel they can trust us to keep confidences and to respect their boundaries when they set them.
A journal is clearly private and should be treated as such - despite the temptation.
The second reason is how to know what to make of what she has written and the importance of recognising it for what it is. Private yes, but also just her thoughts. The whole point of journaling is to get your thoughts outside your head because it helps, and as such reflects a stream of consciousness, her thoughts written down.
That's different than what she might otherwise say, or consider doing - and it's important to recognise that when trying to know what to make of it.
Should you talk to her about any of this? No. Again, not unless there was such a strong concern about risk - because otherwise you risk blowing the trust and damaging the relationship, for what purpose?
Therapy recognises this trust as central to the whole deal, and as such clinicians only very rarely breach privacy for matters of safety, because we accept that to do so we have to be sure that it is potentially worth the risk to intervene in serious circumstances.
Of course, it doesn't mean you can't do anything, and actually reading her journal didn't really tell you anything you didn't know - you already knew she was struggling.
So talk to her about that. Be open with her about your concerns, from a place of empathy, warmth and concern. Let her know that if there is anything she wants to talk about she can, without fear of judgment or punishment.
And if she does approach you, make space and time to be available. Structure time with her, do chores, or family activities together and drag her along. Knock on the door of her room, and check in - by gently asking if she's okay
Because while reading her journal is most definitely too intrusive, you do have to be a little bit intrusive with teens because, left to their own devices, they can drift away. But despite what she says, despite what she does - and despite what she might write in the privacy of her own journal - she still needs you, even if she won't admit it.