The more independent you are as a person, the less likely you are to feel it. Love is a kind of interdependence, so if your "other half" is somewhere else when you're used to seeing them all day, you will feel empty and unfulfilled until they appear.
It's part of our evolutionary response to being alone (and this is why the feeling is akin to general loneliness as far as the physical sensations go), which is generally more dangerous than sticking with others.
The feeling is easy to describe. It will register in different ways or different people, but it's essentially an unease in the guts, down around your stomach and intestines. It is like a combination of worry and loneliness (because irrational worry about a partner would have forced men in particular to keep looking for the women in a tribe to ensure the safety of babies).
The feeling can be intense or dull, depending on your intensity of connection to the person, how long you've known them, and the nature of where they are (obviously a death is going to hit home much more than a day without a lover, though you might not agree at the time due to the feelings being irrational).
So in short, if you are an independent or apathetic type, and you don't really need others, and are used to being alone, you will find that these feelings are very rare for you. And due to them not being defined by past experience, you may also not recognise them for what they are, if they do come.