When You Want What You Can't Have

Isn't that breaking a 10 Commandment? or committing one of the 7 cardinal sins? Coveting? Something like that.
But is it?
I'd like to think that maybe, just maybe, it's ambition. We want something - say to be in a movie based on a book written by you - and it just seems impossible. Absolutely impossible. Do we just write it on our list of "Want Tos", tuck it away, and never look back?
Most of the people I know, don't do this. They might write in on a list - I know I do - but then they start se...