“Planning usually is initiated for the enterprise by your boss, and you’re brought in to fill out the details. […] Planning is like this: Take two steps forward and one step back. It’s like a recursive procedure: You have to continually drill down on a big plan to flush out the details that will eventually be your job to manage.” (Herding Cats, Rainwater)
So when your boss asks you to plan a project, she has already begun the planning. At least the part that connects the project with the company’s overall strategy.
Your job is to take this first plan and develop it, pouring the deeper knowledge that you have, both on technical resources and in people, into it.
Then she (and the other stakeholders) will see your plan, will criticize it and add the business part (time to market, competition, long term goals…) you couldn’t see in the first place.
Finally you will re-plan and iterate through this process as long as needed.
So if someone puts your plan down, don’t take it as personal. Remember that every plan is written to be changed.