[[WMG: Diana actually dies at the end.]]
In ''Light'', Diana actually says "you don't have to be happy ''or'' alive" not "you don't have to be happier alive". Shortly after she abandons them, she commits suicide, making it even more of a DownerEnding. If the line is "you don't have to be happier alive", Alice Ripley pronounces it very oddly in the OBC, suggesting she also believes this theory.
* At the risk of being a killjoy, the line is actually "You don't have to be happy at all/To be happy ''you're'' alive," which can certainly sound like "happy or alive" if you have terrible diction like Ripley does. But even the mondegreen version of the lyric isn't all that suicidal in context (although it does contradict itself - you don't have to be happy to be happy?).
[[WMG: Gabe is actually a ghost, and not a hallucination.]]
This'd explain why no amount of therapy will get rid of him.
* And why Dan appears to see him toward the end of the show.
** And why he can deliberately place drugs where his sister can see them.
*** He's really just a metaphor for shit going wrong in the family. In "I Am The One" he appears as frustrated as his father is (embodying even the sexual frustration, yes) and his mother imagines him as something that she wishes Dan to be, defending her and coming to her aid etc. It's pretty obvious in "Superboy and The Invisible Girl", too; Natalie is not actually being mocked by Gabe, she's just always haunted by this idea that her mother will always want Gabe and not her living child to the point where it could be right behind her and out of sight, laughing at her. You can see in "Catch Me I'm Falling" when Natalie reiterates "she's not here", because she's still thinking about it. Also in the Off-Broadway version when Gabe first comes to see Dr Madden with Diana and tells her that it's not safe and can cause brain damage. It's most likely a voice in the back of Diana's head that she wants someone to have to make her feel better or even stop her. There are other examples, too.
*** Technically everything about a musical is metaphorical (or at least in this one). There's no real in-universe reason that these characters sing when they're not on stage in the story. It's all metaphorical for their dynamics, feelings and arcs. Gabe is obviously a metaphor, but that's not really saying anything. He's most-definitely a psychological representation of the characters' emotions, particularly Dan and Diana. Notice how no one really ever comes into contact with Gabe despite them, as they are the only characters in the story who ever got to know him).
[[WMG: Next to Normal is a DarkerAndEdgier musicalization of ''Series/TheMiddle''.]]
Diana = Frankie
Dan = Mike
Gabe = Axl
Natalie = Sue
[[WMG: Diana told Natalie she was leaving the family right after Maybe (Next to Normal).]]
This explains why Natalie goes right into helping her dad at the beginning of Light, and why she seems more resigned than surprised.