@Juan.
Technically, that's correct. I think that it depends on what is occurring around the sentence.
PAST IMPERFECT: He was thinking about giving up.
PAST PERFECT: He had thought about giving up.
PAST PLUPERFECT: He had been thinking about giving up.
For example, if the action is immediate, the character is in the middle of putting a puzzle together and having difficulty and the writer chooses past imperfect...to me that is passive. It's not immediate and it doesn't read well. The same for the other two forms.
Now, if for example the writing surrounding the sentence is exposition. The character kept coming back to this puzzle, but he didn't know why even bothered anymore. The truth was, it was impossible. At least, impossible for him. No, he was thinking about giving up. That's not passive.
In most cases I believe it's all about context. In general, the fewer words you can use to convey the same message the better. Also, depending on the type of story and the action going on, shorter, concise sentences can really bring out the writing. The last thing we writers want to do is put any stumbling block out for the readers. Especially for aspiring authors, as sometimes all it takes is one sentence to make the agent/editor put the book down.
Just my two cents.