Needed: a Sharp(ie) Retort

I need some help forming a proper response.

Because I went to engineering school, and then became an engineer, I developed the habit of always having a pen on me.  There were far too many occasions where I needed to write to not carry one.

Because of needing to label samples at work, shortly after graduation, a "magic marker" joined the pen as a permanent pocket partner. 

When I arrived at parenthood, the need to put names on various labels and toys substantiated this need.

Here is where I need assistance.

Multiple times when interacting with others in non office public locations, someone has said:

"I need a sharpie."

At which point I pull mine out and hand it to them.

At this point, the person who was looking for one invariably, incredulously,  says some version of:

"You always carry a sharpie with you?   WHY!!??!"

I'm not sure if my reply should be:

"For just such an occasion, doofus!"

or

"Because pinheads like you don't!"