It’s true that when people strongly desire something, such as a new shirt, they might feel like they ‘need’ it – but they don’t need it in the same way that they need water or food. Their survival isn’t at stake.
Desire (as opposed to need) is an intellectual appetite for things that you perceive to be good, but that you have no physical, instinctual basis for wanting – and that’s true whether those things are actually good or not.
The 13th-century philosopher and theologian Thomas Aquinas wrote that these intellectual appetites are part of what has traditionally been called the ‘will’. When people want something, they strive for it. If they come to possess the object of their desire, their will finds rest in it – and they are able to experience joy, so long as they are able to rest in the object of their desire.