(These answers are meant with humor and a tad bit of sarcasm)
Do not ask a single girl "why" she is still single, this may result in many ridiculous responses or a slap across the face. The question is really akin to asking a single girl "what is wrong with you" and trust me... She will respond appropriately, likely with sarcasm..
Do not tell a single girl you "understand" why she's single.. Please see the previous paragraph for her likely responses.
If you want to make a single girl feel pathetic, tell her you feel sorry for her still being single, that is not helpful nor will it make her feel better... Plus, what will you do if she cries?.. Feel like a jerk.
Telling a single girl that her "prince charming is out there somewhere" only makes her want to kill the makers of every Disney princess movie ever created. Especially if she thinks "prince charming" tends to be a sort of boring fellow in fairy tales...
Explaining that a single girl's "dominant" personality might have something to do with "it".. is like claiming she's single cause she has brown eyes... It makes no sense, and only proves that you also have no idea why she is still single.... other than the obvious fact that she has not gotten married yet.
Just because a girl is single does equate her also being lonely.. It makes her.. single, that is all. Do not assume one inherently means the other anymore than getting married solves all your problems.
Single girls really can (believe it or not) just be friends with guys (single or married). IF she has had the "could I see myself with him?" thought about a single guy, that does not mean she ceases to be "just friends," it means she's realistic and not shallow...
...If the single girl has not wondered if she and a single guy would work together, that does not inherently mean she is being shallow, she likely knows herself well enough to know whether they would work well together. Both scenarios are good things.
Believe it or not, high standards is actually a good thing (for guys and girls). Do not try to convince the single girl otherwise... Her dad won't appreciate it.
Telling a single girl to "enjoy" being single while she can, is like telling someone to enjoy their freedom before they are sent to prison... Aka... You have just indicated that you feel like a prisoner, and she will feel sorry for you and your marriage.
To be continued...
Feel free to add... ;)