energy

In the classical view, one of the big drives in evolution has been getting enough energy for the organism. If you can't get enough food, you die. Of course, other things are bad for your fitness, too, like getting eaten or not finding a mate. But your energy input also limits your other qualities. Organisms have to balance their energy investment between muscles, brains, reproduction, and other things. If you had enough energy, you could outrun all your predators and prey, develop elaborate mate-attraction anatomy, develop super sensory organs to hyper-process every modality, and not have to compromise anything.

Nowadays, humans in the developed world essentially have access to unlimited food. The problem is that our biology doesn't know what to do with all that energy. Enzymes have ranges of substrate concentrations that they work in; gene transcription has programmed ranges of outputs; and most importantly, the structure doesn't exist to use the energy. For example, if you could always count on plenty of ATP being available in the cell, you could manufacture a new protein that would do something awesome (like shooting X-ray beams to kill enemies).

Throwing a lot of energy at a system doesn't immediately make it work better. All it does is provide a level of support so if the system invents a new structure that requires energy, that structure won't necessarily fail.

Same with people. You can't improve people by immersing them in love; but if you do provide that energy, they won't be clobbered down when they let go of some of their defenses.