Soft Spot Meaning

(idiomatic) A sentimental fondness or affection.

Example:   He started out a bricklayer and throughout his life he had a soft spot for other bricklayers and masons.
2011 December 15, Felicity Cloake, “How to cook the perfect nut roast”[1], Guardian:
  It's a shame; not only are nuts quite ridiculously nutritious but, as anyone who's ever shelled out (sorry) for a tiny glass of pistachios in a pub will know, they're a guaranteed crowd-pleaser. But although I happen to have a soft spot for nut roast – an option often preferable to the meat that emerged from the school kitchen – it seems I'm in a cranky minority. A request for recipe recommendations was met with a polite silence on Twitter: vegetarianism, apparently, has moved on a bit. You don't see Yotam Ottolenghi faffing about with nut roasts, do you? But I'm determined to revive the fortunes of this much-maligned classic. After all, Christmas isn't Christmas without a luxury nut selection.
A point of vulnerability in a defence
A fontanelle