Can't Help But

The can't help but construction (with other forms of the verb, like cannot and could not) is a little illogical: it comes from two other constructions, can't help —ing (meaning “I can't keep myself from —ing”) and can't but (meaning “I can't do anything except”). So can't help but should mean “I can't keep myself from doing anything except,” which is a kind of double negative. Still, can't help but has been around for a long time (the OED traces it to 1894), and it's probably not going away, so it's not worth grousing about. I avoid it myself, preferring “can't help —ing,” but there are better things to worry about.