Whenever I smile at someone, they look away...?

It just means they don't want you to think they are flirting with you or checking you out, either because they really aren't interested in you and want to prevent any advances, or because they don't want you to think they are. Consider it as nothing more than they are being respectful towards you, especially if it's in a workplace. It could also mean they were caught off guard or embarred or their mind is off in la-la-land and they aren't paying attention.