Is it the case that one who abandons the forbidden out of fear of diseases, and not out of fear of Allah, feels the same happiness and expansion of the chest in this world as one who abandons it out of fear of Allah? And does a Muslim differ from others in this feeling?

A person is rewarded for abandoning the unlawful (haram) if they do so for the sake of Allah; otherwise, there is no reward for them, even if they are spared the sin. Abandoning sin is one of the causes of happiness and a sense of ease, and this is specific to those who abandon it for the sake of Allah, as stated in the Hadith Qudsi: "If he abandons it, then record it for him as a good deed, for he abandoned it because of Me." And it may be that one who abandons it for reasons other than Allah receives some happiness or a sense of ease, for Allah gives to everyone in this world, as Allah the Almighty says: "To each – these and those – We extend from the bestowal of your Lord. And never has the bestowal of your Lord been restricted."