Does enjoying the sweetness of worship, or feeling comfortable while performing it, or self-love in being a worshipper instead of a disobedient one, invalidate worship and contradict sincerity, especially if the intention of seeking God's pleasure or expecting reward is absent?

The love you feel for worship is good and does not diminish sincerity; rather, it is one of the signs of faith. It was reported that the Prophet, peace and blessings be upon him, found the coolness of his eyes in prayer.

The Sharia encourages good deeds by explaining the good life they contain in this world. There is no harm in combining the intention of worshipping Allah with the aim of a good life through these acts of worship, unlike intending worldly pleasures that contradict worship, such as showing off (riya').

The pleasure you find in worship is a blessing from Allah that deserves gratitude, and it helps in adhering to it. Any thoughts to the contrary are from the whispers of Satan.

The various intentions for worshipping Allah, such as glorifying Him, seeking His pleasure, and finding solace in Him, all lead to one ultimate goal: desiring Allah alone, and this achieves sincerity.