You.

 I am not sure what exactly the metaphor means.
But here's my theory.
(Note that I am full of theories)

At times I can be very happy-go-lucky,
at times I can be very moody and refuse to talk.

But there'll always be someone that
can give me the "roller coaster-that-only-goes-up" kind of feeling.

That one person that knows
how to make me feel moody
and how to make me feel better again.

And if you're reading this,
you would know.