Similar to what Zeizie999 said, it has to do with potential consequences.
I'm not personally of the karma or "threefold rule" schooling, but I do believe in cause and effect. It really depends on *what* you're doing for "personal gain". Magic, in my opinion, follows its own form of physics (for lack of a better term).
Example comparison:
In this plane (good old meat space, so to speak), a person enters a convenience store and steals something from the shelves. Let's hypothesize that they get caught: the owner of the convenience store presses charges, shoplifter is found guilty, sentencing declares the shoplifter serve, oh, 30 days of community service or somesuch.
Clear case of: steal for personal gain, get caught, personal repercussion, pay price.
Say the shoplifter doesn't get caught: they've still violated a rule of society and have caused the shop owner to lose money. If the shoplifter continues, they will likely get caught at some point and pay the price. If the shoplifter believes that shoplifting is wrong, then they are also living with their reaction to doing something that is counter to their belief system. Effects on that vary wildly so I won't try to speculate on that score, but there are still tangible effects.
In the magical plane (pick one), a person decides to steal energy from another entity (forcibly taking the energy without permission or agreement). Hypothesize that they get caught immediately by the intended target: reaction by target varies depending on nature of the target, but there is likely to be *some* reaction and consequence usually ranging from a mental slap on the wrist to anywhere from equal stripping of power of the potential thief to (in my opinion) over reaction of stripping full powers/abilities from the would-be thief. Hypothesis: Thief gets caught by a third party force: varied reactions, again, (we all have different motives and varied ethics, reactions, etc.) but usually within the same range as the previous hypothesis although if the third party entity happens to know the thief in meat space, then there might be reactional consequences to pay there as well.
In social and magical settings, the physical physics rule, "For every action there is an equal and opposite reaction," doesn't necessarily hold true. Humans, souls, spirits, entities, etc. are not (usually) as predictable as physical physics. However, there are usually reactions to actions.
Thinking through what the possible effects of your actions are and making allowances for mitigating the worst of the possible effects is generally a good practice. Having an idea of what you're willing to live with in terms of potential consequences of your actions is a good way to problem solve for what you are and are not okay with doing in your magical practice.
