"You shall not steal."
We all steal. Perhaps we call it borrowing without asking, or we have some other clever name for it, but regardless, taking something which we have no right to take is in fact, stealing. There are many excuses for stealing, such as "It was just going to waste anyway" or "No one is going to miss it" or even worse "I deserve it because of..."
Stealing is hardly considered a major offense today, but it is included with commandments prohibiting murder and adultery and carries the same consequences. What's the big deal? While the damage may be minimal to those you steal from, the self-destructiveness of stealing is catastrophic. Stealing and dishonesty go hand in hand and those who steal tend also to be liars (in order to cover up their thievery). Stealing is a gateway sin which leads to many others (covetousness, pride, deceit, greed, etc.)
But "He who has been stealing must steal no longer, but must work, doing something useful with his own hands, that he may have something to share with those in need." (Eph 4:28) The opposite of stealing is not "not stealing" but rather generosity. We are called to lives that give of themselves selflessly as a reflection of our generous Lord. Stealing is a denial of the image of God we were made in and an affront to his glory because we are not able to reflect Him properly.