[Outdated, please look at pinned post] Casual Conversation

One of -- if not THE -- primary causes for attraction is proximity. If you see someone often, you are much more likely to become attracted to them. Family members and 'unavailable' people such as those already married are typically, but not uniformly, excluded.

After that, we tend to be initially attracted to pretty people with symmetrical features, good health, and of a similar social status (we are also attracted to those of higher social rank, but they will tend to self-select themselves to be less frequently proximate as well as rejecting overtures from potential mates of lower status). That still doesn't matter as much as frequent exposure to someone. Ideally the exposure occurs when you are both in a good mood. Bad moods make for less attraction. We also like people with whom we share common interests, habits, and so on, such that more similar people are more likely to become attracted to one another.

So, yeah, 'friends' are generally going to trigger psychological cues of attraction in any group. Most everyone has to deal with such feelings and quash them when appropriate. Some people have a hard time dealing and either pursue when that makes them creepy or they fail to respond when the feeling is mutual.