There are two kinds of people in the world, the 'why' people and the 'why not' people. 'Why' people are negative. You tell them you want to do something and they say (verbally or with a condescending facial expression) "why do you want to do that?" just because they don't understand your reasons. 'Why not' people are far more positive and say "Sure, why not?"
"I want to make a movie (or some other creative project)." (nose crinkes up) "Ew, why you wanna do that?" "Because it's fun!"
Or..."I want to make a movie." "Sure, why not! Sounds fun." To a 'why not' person "because it's fun" is a legitimate, satisfactory reason 'why', so they say "why not!"
Now, this is not to say that 'why not' people don't care about consequences or will do anything just because. If there are legitimate reasons not to do something, a 'why not' person can still be intelligent and realize the answer to their own question. "Why not?...oh, that's why!" They may seem like a 'why' person to someone who wants to do the foolish thing, but only because they asked 'why not' FIRST (being positive) and came up with good answers to said question.
"I want to smoke crack." "Well, why would you want to do that when (insert smart reasons not to smoke crack here)."
Notice that even though the 'why not' person is asking 'why', it's only because he has collected some reasons 'why not' and is sharing such. He is still a positive thinker. He just feels that your course of action is unwise. There is a BIG difference between that and someone judging your course of action and saying "why you wanna do that?" just because they don't get it.
The above examples illustrate how 'why' people can bring you down in creative endeavors, but they are particularly dangerous in spiritual matters. (Mark 8:31-33) Imagine a young person telling a non-witness 'why not' person (maybe a relative) that they want to go to Bethel and the person says "Sure, why not! It's something you believe in. You're young. Volunteer work is good for you. They train you, right? Go for it!" Even if they weren't witnesses, didn't understand the spiritual benefits and/or had concerns about the young one making a living, they were still positive. "Sure, why not?" It might be rare for a non-witness to think that way, but it illustrates how a positive 'why not' person can be a good influence on someone, how much better it is to surround yourself with 'why not' people and how inexcusable it would be for a witness to be a 'why' person when someone has spiritual goals, to pioneer etc. "You wanna pioneer? Why you wanna do that?" vs. "Sure why not? (pause) Oh, you got reproved last month? Well, um, I guess that's a reason why not, but once you get your restrictions lifted--why not? Go for it!"