Here are some examples of how it's used:
* "I can't just pick and choose the assignments I want to do. I have to take what's given to me." This shows the negative connotation of being demanding.
* "You can pick and choose from our menu of delicious appetizers." This highlights the positive connotation of selecting carefully from a variety of options.
Essentially, it implies a degree of selectivity and implies that the person choosing has a lot of options.