Everything on earth absorbs energy from the sun. Plants absorb energy through bark and leaves; buildings through siding, windows, and roofing; and mammals absorb energy through their skin.
All mermaids and some (maybe most) human females re-emit this energy as happiness rays. Clothing and fabrics, for the most part, block the happiness energy, but there are a few coverings that magnify the effect.
Mermaids, especially, are able to emit huge quantities of happiness rays, not only from their skin but from their teeth, hair, and eyes. Those not used to the effect should use caution in the presence of mermaids.