St. Thomas writes that "joy is compared to desire, as rest to movement… and rest is full when there is no more movement. Hence
joy is full, when there remains nothing to be desired.” So God alone can give true joy.
The pure of heart, inspired by grace, seek God alone, so they infallibly receive what they desire. In a
spiritual sense, then, they are full of joy. “The greatest joy which it is possible to taste on earth," writes Sr. Consolata Betrone, “is to possess God, God alone.”
In another sense, it is possible that one is subject to weariness, temptations, trials and sufferings of all sorts. Think of St. Gemma Galgani. She possessed God; she was intimiately untied to Him, and conversed with Him freqeuntly. But she was also attacked by the Devil, tormented by temptations, and filled with sorrow at the sight of her own sins and those of others. Upon hearing blasphemy, she sweat blood!
It seems to be the case that those who love God the most, possess a deep joy and peace; but they might also experience suffering in their lower nature i.e. in their passions.
It is difficult to offer a comprehensive answer without writing a short essay
