If you say a sentence but only one word in that sentence sticks out to you, that word will dominate the meaning of that wish. If you say I don't like spinach, and have a serious distain for spinach, spinach will carry a lot of weight the rest wont carry enough to matter.
When you "say" I don't like SPINICH" but have a very negative and redundant thought stream about it (thinking "why you don't like it" is normal, and this triggers visualizations which carry a tremendous amount of vibrations).
The words carry a certain weight on their own but the trailing thoughts about it are what really push the sentence into submission (like afterburner jets) with a negative result returning. The universe doesn't hear negative or positive it only hears wishes, requests, orders, and reads the energy output (raw power/vibration value) so what has the most power gets precedence.
Also realize it's the definition you personally give it (often subconsciously) that makes it what it is. Remember the universe doesn't know words or their meanings, it knows only what you fuel it with (definition based on perception charges it with that energy).
By energy I'm not talking esoterically, this is a shortcut term to describe the complex nature of the subconscious mind and how it reacts (using your body) with the outside world causing ripples.
If you practice being positive in your talking, thinking, writing, and any other area where your thinking, then the habit will form protecting you from negative wishing pitfalls.
The universe is not judgmental, other than it responds to energy. In the spinach sentence example above its entirely possible that the word spinach will be so powerfully backed emotionally (even if negatively) that the universe will work hard to BRING you spinach!
You will go to a friend's house and it will end up on your plate! You think to yourself I said I DONT want spinach (then in this sentence the don't is highlighted and becomes the message to the universe.) but a little too late huh?
Don't ever say I don't want a mean girlfriend! Say I want a compassionate and loving girlfriend and emphasize all of the words. A "word Spell" is powerful if you know how to cast it. The Secret DVD touched on this with some detail but this article expands on that awareness to a degree they didn't explain it fully.
Two sentences side by side containing two emphasized words can be dangerous as the two words could be carrying the same amount of energy, trailing ahead of the other words, and thus become a new sentence as far as the universe is concerned. See how this energy thing works? Energy can be intention and subconscious thoughts (which always translate into actions in the outside world, or reciprical actions).
It's not what you ask for but how you ask for it and how much control over this you have. Learning to balance your words out so you get what you want not what you fear is essential to the process!
You could calibrate what words mean and how they affect you when you say them (in a wish) by watching your results. If always wishing "I don't want" or "don't let such and such happen" brings you negative results you can figure out that your asking wrong.