Might have to give more details here... they tend to be pretty reliable in my experience, even in the bad old days, they didn't have many moving parts--and today, none.
Any chance you drive with both feet? One for gas, one for brake? Any time the brake light switch is activated, cruise disengages.
On most (all?) cars cruise has a separate switch to allow it to be used. An on/off switch that has to be put to "on" before the "set" button will do so something. [My old VW had a slide switch for it, and the only times it was off was after the few times I took it to the dealer for anything. But every other car I've had, darn thing has to be turned on each time. Hard life I know.]
Few years ago I tried using cruise in a Prius in a 40mph zone, and the foolish thing had the nerve to warn me about paying attention! nearly did an LCD delete right then and there--but it did engage.
There might be a min speed for cruise to work, like you have to be doing more than 30mph.