Everybody has slightly different opinions on the relationships that they have with their colleagues. Some like to see them as an extension of their social circle, people they can rely on to do the occasional favor and hang out with outside of the office now and then. Others prefer to keep it strictly professional, as they are wary of the complications that can come from being too close to their coworkers. Also, it's not like most employees have much of a say in who they work with. They might just not like them very much.
An awkward situation that is not uncommon in the workplace is when two coworkers have different ideas of what their relationship means to them. One wants to take it further, whereas the other prefers to have boundaries to stop them from getting too close. Neither of them is necessarily in the wrong, but it can create friction if it isn't addressed before it turns into a big problem. Those who don't want to be friends need to make it obvious that they aren't there to socialize.