This is an American joke. A woman who gets asked out on a date and doesn’t want to go with that person might say, “I can’t go to the dance with you. I need to wash my hair.”

In other words, it is such a ridiculous reason to not go to a dance, it’s obvious that the REAL reason is, you don’t want to go to the dance with the person who asked you.

This fits SuStel’s interpretation because it doesn’t matter whether the hair is clean or dirty. Maybe there’s a change from being dirty. Maybe there isn’t. You need to wash it so you don’t have to go out with the person who asked you. It’s even better if you imply that washing it would not cause it to be any cleaner than it already is.

That's not the reason.

vISay'moH
The cause of my hair being in a clean state is me.

vISay'choHmoH
The cause of my hair going from an unclean state to a clean state is me.

vISay'nISmoH
The cause of my hair being in a clean state is me. I have a need to cause this.

vISay'nISchoHmoH
The cause of my hair going from an unclean state to a clean state is me. I have a need to cause this.

DaH jIbwIj vISay'nISmoH
I now have a need to be the cause of my hair being in a clean state.

It has nothing to do with offering insincere explanations. It is simply not important to the speaker to express a change of state of his hair. Needing to be the cause of his hair's cleanliness is all that matters. If someone you genuinely wanted to be with asked you out to dinner, but you actually had scheduled a shampoo at this time, you could say this sentence completely sincerely, and you wouldn't need -choH. The point is not to express a change in cleanliness; the point is only to express that you're engaged to be the cause of cleanliness.

Now, if my son were to get mud all over his hair, I might tell him, DaH jIblIj yISay'choHmoH Get your hair clean now! Here, the change of state of his hair is very much the point of what I'm expressing.

-- 
SuStel
http://trimboli.name