What are the reasons for being single over 40?

There is a lady at my work who is mid to late 40s, lives with her dog. Doesn't seem to have much of a social life. Outside of taking her dog for a walk, doesn't have much else to say. I find it unusual because she is actually attractive, highly intelligent and friendly to all and I'm considerably younger.

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Sorry, should mention she has never been married and has no kids.
What are the reasons for being single over 40?
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