The problem with a long-distance relationship is that there is only one way you can spend time with each other – by talking to each other. Of course there are many ways of doing this now, given the advances in communication technologies – text chat, voice chat, video chat, …
But as you can see, the common thing to all this is chat. And to chat, you need things to chat about. Which means that the amount of time you spend with each other is limited by how interesting your days have been and what you have to say to each other.
It is a rather common occurrence that you want to spend more time with each other than what what you have to say to each other dictates, and in such times, you have a few options.
You could talk about everyday happenings like the weather or some news or sports (extremely unlikely, though, that a couple will spend time discussing sports). Another popular option is repeatedly saying lovey-dovey things such as “I love you” or “I miss you”, so that you prolong the conversation. You could even tell each other the mundane details of your lives that day, like “and then I got into bus 37 and … “. Else you could talk about your respective mental states, like “I got so psyched out while writing this algorithm this morning” or “I felt so happy I answered that question in class”. And so forth.
While all of these make for excellent fillers, and help you spend more time with each other by way of creating things to talk about, the problem is that they seldom add value (except perhaps for small doses of lovey-dovey talk). And when you over-indulge in filler conversations, they end up subtracting value from your conversation by overemphasising attention on the mundane at the cost of talk with positive information content.
Contrast this with a “normal” relationship where there are so many other ways of spending time with each other other than talking – I don’t need to enumerate them here.
In other words, when you do long distance, you only have access to a small subset of your relationship. Which makes long distance hard. And occasionally makes you go mad.
PS: tools such as FaceTime allow you to “virtually be with each other” by dialling and then going about with your own lives. But you are still stuck to the fixed point which is the computer, and that means whatever life you go about is unreal, and that can further add to the pressure! And hence subtract value.