Why are so many high functioning happy people drawn to the sad ones whose lives are a mess?

It depends what you mean by drawn.

High functioning people, by definition, have high empathy and a desire to see others flourish.
They will help as best they can as long as it doesn't cross boundaries.

But someone who seems functional on the surface yet chooses an intimate relationship with someone who is highly dysfunctional is usually a codependent.

The ones I have seen that are usually just outwardly happy, and try to fix the unhappiness of others for some intricate psychological reason.

I seem to attract that and I hate it.

Some people are "fixers".

Because we think we can help them.

I'm more prone to negativity & wouldn't mind someone like me or more positive. I think as long they don't push positivity on me.

I attract deficient fuckers no one else is even nice to then I help them and they hurt me.
I'm protective because no one protected me.

They actually approach me, because I guess they think I can help.

Sometimes that is merely a façade. The underlying is different.

pity, maybe?

There are people of both genders who feel the need to “save” folks; [b]they[/b] need to be needed. So they’re not quite as functional as they appear, either, or they [b]wouldn’t[/b] be drawn to “broken” people.