On the other side someone that's never been in a committed relationship can't relate to the everyday troubles of those who are. And there are very few things in life that have any real similarity to marriage, some form of equivalency a person can use to relate. And none of them are on the standard path to becoming a priest.
You have not lost someone close to you, but you have experienced suffering- you know what it is like. Furthermore, you know from other sources (or could know if you wanted to) what it is like for people to lose a loved one. Take for example C.S. Lewis's book A Grief Observed. It provides unique insight in the experience of losing a loved one. Lewis allows you to have a look at what he has experienced subjectively. We can get similar insight from other, books, movies, plays etc. We can also get insight from those who we know. When your friend loses a loved one you are in some ways party to it. You grieve with them. All this points to the fact that we can have genuine insight into that which we have not experienced directly.