To act out patterns usually feels good. We give our patterns what they want and so we feel like we are getting what we want: such as when we act in anger and get the other person to pay attention and listen to us.
Enmeshment is very simple: the actions of another cause certain feelings in us which we find unpleasant. We then react, either by overt provocation and confrontation, or by "withholding," withdrawing emotionally (sulking) and so controlling the other through creating distance. The aim is to generate feelings in the other that match our own.
This happens when honest and open communication breaks down, for whatever reason (usually our own fear of vulnerability and exposure).
Simply put, if we feel angry about what another does to us, instead of either taking the anger away and processing it alone, or communicating the anger to the other openly and softly, we try to provoke anger in the other so that they feel the same way we do. We have then "communicated" our feelings to the other, in the least honest (yet also most "direct") fashion. This is also known as "off-loading."
Letting our patterns control us means we get the "feel-good" of indignation or self-pity; but instead of being driven by patterns (which is like the donkey trying to escape the whip), we can give in to them without acting them out. That giving in causes us, as awareness, to sink deeper into the patterns, into what's beneath them. Beneath anger is usually fear of some kind; beneath self-pity a deep sorrow, and so on.
This doesn't feel good, because we are going into the very experience that our patterns were designed to take and keep us out of. But once we allow that experience, our patterns begin to dissolve: because they are no longer needed.