Why Does My Guitar Keep Untuning?

This could be due to a variety of things, including: strings needing to be stretched, or seated firmly in the various areas that they are in physical contact with. components on the guitar itself needing to become firmly seated (loose neck bolts, bridge, tuners, nut, etc.) quality of the instrument.

Why does my guitar detune?

Each string moves through a slot in the nut at the top of the fretboard, and if they're not moving through that slot smoothly, the result is tension on one side or the other. If the strings don't sit in those grooves along the nut's front edge as they move down the fretboard, you'll be out of tune throughout the neck.

Is it bad to leave your guitar pick in the strings?

If you want to keep your pick in the strings (I've done it for years...), do it. It will not hurt your guitar a bit and if it happend to effect your tuning a micro cent or so, they put these knobs on the headstock.....