fondness vs soft spot

fondness and soft spot both are nouns.

Word	Noun	Adjective	Verb	Adverb
fondness	Yes	No	No	No
soft spot	Yes	No	No	No

As nouns, soft spot is a hyponym of fondness; that is, soft spot is a word with a more specific, narrower meaning than fondness:

Other hyponyms of fondness include attachment, fond regard, protectiveness, regard, respect.

fondness (noun)	soft spot (noun)
a quality proceeding from feelings of affection or love	a place of especial vulnerability
a positive feeling of liking	any membranous gap between the bones of the cranium in an infant or fetus
a predisposition to like something	a sentimental affection