From Letter No. 43, to his son Michael:
A young woman, even one 'economically independent', as they say now (it usually
really means economic subservience to male commercial employers instead of to a
father or a family), begins to think of the 'bottom drawer' and dream of a
home, almost at once. If she really falls in love, the shipwreck may really end
on the rocks. Anyway women are in general much less romantic and more
practical. Don't be misled by the fact that they are more 'sentimental' in
words – freer with 'darling', and all that. They do not want a guiding star.
They may idealize a plain young man into a hero; but they don't really need any
such glamour either to fall in love or to remain in it."
Tolkien's worldly side, straight from the horse's mouth. Eowyn simply
recalculated her prospects. :)
Or it could be true love. That's why it's a fantasy.
:)