Absence makes the heart grow fonder

Proverbial Meaning: Being apart can make a couple's love stronger. Or, not so much in modern use, friends and acquaintances seem nicer when you haven't seen them for a long time. 

Literal Origins: When people are absent from out lives we often forget all their bad points and only remember all their good points, or their bad points don't seem so important when you realise you miss them for their good qualities, which outweigh the bad. 

Common Usage: This proverb is very well-known and often quoted directly to cheer people up who are about to be separated for a long time from their boyfriend or girlfriend. Or sometimes the couple may say it to each other as a comfort.