Does the practice of deeds that the soul is accustomed to and fond of contradict sincerity, making them mere desires and whims instead of being for the sake of Allah?

Loving acts of obedience and feeling at ease with them are indicators of goodness and do not contradict sincerity. A Muslim should increase in the acts of obedience that he is drawn to, study books of encouragement and deterrence, and ask Allah Almighty for the love of acts of obedience and help in performing them, just as the Prophet, peace and blessings be upon him, asked his Lord for that, and he informed of his love for prayer, and that it was the coolness of his eye.