What gossipy and catty people say about a woman whose friends are all getting married yet who remains single herself.

Is she ugly?
Is she misandrous?
Is she waiting in vain for that man she met a year ago who said that he'd call her but hasn't?

Probably not. She just hasn't found the right man (or woman). But until she does, people will talk about her behind her back. Those bastards.

Also a colloquial way to indicate that someone is often in contention for rewards or other good things but somehow never pulls down the big prize. Can also be used sarcastically:

"Morning, Boss. Where are Judy and Syed?"
"Laid off. But don't worry, you still work here."
"Always the bridesmaid and never the bride!"