I choose it according to its affordability, applicability to specific projects, comfort, color and texture.
For example, if I want to make a scarf, for people living in a real winter climate, I will choose heavier yarn (worsted or roving), containing some wool but not 100% wool, because using 100% wool happens to make my fingers itch.
If it is for children, I will choose cheaper yarn that meets these parameters, because children are easy to lose things, and I don't want them to lose expensive things.
If I make it for adults, I can choose something more expensive, because I know that adults are less likely to lose it, and I prefer to wash it by hand when needed, or follow other washing or drying instructions.
Therefore, I choose yarn partly based on the expected recipients of the project. I will also choose it on time to a certain extent - if I am making hats for charity and need to complete 10 hats in a month, I will choose roving; If it is for my own sake and there is no deadline, I will choose the yarn with lighter weight, and the effect will be slower in the smaller stitch.
