For example, in how many movies, as an audience member, have you watched someone enter a dark and scary basement knowing the character was crazy because he/she was going to die, get captured, tortured, fill-in-the-blank… You think to yourself, the character is a complete idiot, call 911, get backup, turn tail and run, anything but enter the basement. As the audience member, you just asked yourself the first “but why?”.
Have enough unanswered “but whys?” in your WIP and it will make the reader shut down in disgust thinking your hero/heroine is stupid.
The good news? It doesn’t take much to fix this unanswered “but why?” It could be a simple thought in your character’s head (if he/she is alone): I really don’t want to go into this dark and scary basement, but the circuit breaker is in there and I really want lights on about now. Motivation now revealed, your character's IQ has jumped by leaps & bounds.
Bob: “Are you crazy? The killer is probably down there!”
Jane: “But we need the lights on, besides there are two of us and one of him.”
Bob: “Screw that, let’s get out of here.”
Jane: “Be brave, we can do this.”
Bob: “Fine, we’ll go down into the dark and scary basement, but if we die, I’m blaming you!”
Oh, and there’s one “but why?” fix you could get away with (probably) just once: “I have no idea why….” the "gloss it over" trick. After all, remember Casablanca?
Signor Ferrari: "...why, do not know. Because it cannot possibly profit me, but..."
Actor, Sydney Greenstreet's line is a classic example of a Plot Hole/ Motivation cover up.