Watch Dan Ariely do a great job explaining some experiments that lead to this conclusion, or read the full paper, The “IKEA effect”: When Labor Leads to Love (pdf), Norton, Mochon, Ariely HBS working paper, 2011.
Turns out, if we’ve put effort in ourselves we’re also likely to think that others will like it even more just as we do—usually not the case. But for us at least, more effort = more love.
Dan points out that kids are the ultimate expression of the IKEA effect: they’re very hard, they don’t come with instructions and they take a lot of effort…
Also see The Betty Crocker Effect, or ‘the egg theory.’