If we read the Words of Allah, the Most High:
"O you who believe! Observing As-Saum (the fasting) is
prescribed for you as it was prescribed for those before you,
that you may become Al-Muttaqun." (Al-Baqrah:183)
(١٨٣) يَا أَيُّهَا الَّذِينَ ءَامَنُوا كُتِبَ عَلَيْكُمُ الصِّيَامُ كَمَا كُتِبَ عَلَى الَّذِينَ مِنْ قَبْلِكُمْ لَعَلَّكُمْ تَتَّقُونَ
We'll explore why fasting is required. It's about Taqwa and worshipping Allah, the Most Glorified, the Most High.
Taqwa means avoiding bad actions and doing what Allah asks, and not doing what He says not to. The Prophet (PBUH) said:
If someone keeps lying, doing bad things, behaving badly, and saying mean things to others, Allah doesn't care if they fast or not.
This shows that when fasting, it's important to do what's right and avoid what's wrong. So, a fasting person should not talk badly about others, lie, spread rumors, or do anything wrong. If they do the right things and avoid the wrong ones for a whole month, it helps them be better for the rest of the year.
But sadly, many fasting people don't act differently when fasting. They keep doing what they usually do, not doing what they should, and doing what they shouldn't. They don't show the seriousness of fasting. While these actions don't cancel their fast, they reduce the reward. On the Day of Judgment, these sins might outweigh the good of fasting, causing them to lose its benefits."
Fataawa arkaanul Islam ibn uthaymeen