Is it permissible for a Muslim to be happy with worldly matters, and what is the benefit of this happiness given his knowledge of its transience and that the purpose of his existence is worship, and is it better to spend one's life solely in worship?

Allah has instilled in His servants a natural inclination to find pleasure in what suits them and pain in what displeases them. Pleasure is an innate matter, and there is no blame upon a person for it. The measure of pleasure refers back to its true nature. The pleasure of the people of faith is found in intimacy with Allah, and all their happiness is in His worship. The fleeting worldly pleasures they enjoy are used as a means to obey Allah, thereby attaining two pleasures: the pleasure of this world and the pleasure of the Hereafter. Taking pleasure in the forbidden is forbidden, while taking pleasure in the permissible is permissible, with no harm in it, unless it distracts one from what is more beneficial. Pleasure is condemned if it is followed by a pain greater than itself or if it prevents a better pleasure, and it is praised if it aids in attaining lasting and stable pleasure. This world is but a provision and a means to the Hereafter. Thus, every pleasure that aids in attaining the pleasures of the Hereafter is beloved and pleasing to Allah, and its enjoyer takes pleasure in it from two aspects: from the aspect of enjoying it, and from the aspect of its leading him to the pleasure of his Lord. The successful person is he who makes all his pleasure in obedience to Allah and intimacy with Him, even intending a righteous intention for permissible things, thereby turning them into acts of proximity and obedience. As for the foremost in faith, the muqarrabun, they utilize permissible things when there is obedience and good intention in them.