While travelling in the countryside and seeing an enormous tree with overarching branches and abundant green leaves, someone whose mind is expansive and appreciates the beauty of nature will delight in the splendour and magnificence of that tree and wish for it to prosper and be free from danger.
In that moment the mind is devoted and directed to the tree’s wellbeing. One’s thoughts are benevolent; they are not selfish, acquisitive, or covetous. The mind delights in the healthy, natural state of the tree. This state of mind is wholesome, virtuous, and peaceful; it is beneficial to that person and to others. The pleasure in witnessing the fulfilment of this tree or the wish for it to exist in a state of completeness is wholesome desire (chanda). One can say that one feels goodwill towards the tree.
In the same manner, we may see other people in good health, strong and at ease, and we delight in their wellbeing, wishing for them to be happy, healthy, and free from illness. This state of mind radiates outwards and does not revolve around selfish concerns. This wish for other beings to exist in a state of happiness and fulfilment is wholesome desire as it is expressed towards living creatures, and it is given the special designation as loving-kindness (mettā).
From chapter 21 of Ven. Phra Payutto’s Buddhadhamma on the Buddhist teachings on desire.