When I sit down to choose my next project, several factors come in to play.
First, I choose a new project based upon my working circumstances. Will this be a commuting project? Then I must choose a portable and monochromatic project. Will I work on this at home in front of the TV or when hanging out with friends? Then it must have a simple repeat pattern. Do I want to sit and really concentrate on something? Then I will choose a thread lace pattern, whether a bookmark or a doily depending on the time I wish to invest. (pineapple shawl)
Second, I might be inspired by some new yarn. The weight and texture may lend itself to one type of project or another. For example, I recently purchased some DK and fingering weight yarns that are fated to become light-weight shawls.
Third, I might be come across an innovative pattern that looks like a worthy challenge. I tend to enjoy lace patterns for the mental challenge they present. Similarly, I participate in an online crafting competition that provides 5 challenges monthly, and these will often inspire my pattern choice - or my own design.