I just reported a video for thieving for someone, however, it doesn't
give you an option anywhere (that I can find) that lets you explain
'why' you're flagging it (which should be optional anyway).
One thing I'd like to see in future on the rare circumstance I have to
report something, is why you're flagging it, and let you explain to
whoever receives flagged videos WHY you're flagging it.
When you say "thieving", are you talking about copyright infringement?
If so, then only the copyright owner has the right to report this.
Also, it can't be done with the flagging system: trying to flag for
copyright infringement will bring up a message telling you to submit a
proper DMCA complaint.