عنْ أَبِي سُفْيَانَ ، قَالَ : قَالَ عَبْدُ اللَّهِ : قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : " عَلَيْكُمْ بِالصِّدْقِ ، فَإِنَّ الصِّدْقَ يَهْدِي إِلَى الْبِرِّ ، وَإِنَّ الْبِرَّ يَهْدِي إِلَى الْجَنَّةِ ، وَإِنَّ الرَّجُلَ لَيَصْدُقُ حَتَّى يُكْتَبَ عِنْدَ اللَّهِ صِدِّيقًا ، وَإِيَّاكُمْ وَالْكَذِبَ ، فَإِنَّ الْكَذِبَ يَهْدِي إِلَى الْفُجُورِ ، وَإِنَّ الْفُجُورَ يَهْدِي إِلَى النَّارِ ، وَإِنَّ الرَّجُلَ لَيَكْذِبُ حَتَّى يُكْتَبَ عِنْدَ اللَّهِ كَذَّابًا "
We can understand from this hadeeth that being truthful and honest guides to righteousness which leads to heaven, and lying guides to immorality, and immorality leads to hell.
Back to your question. Why is lying is such a major sin and guides to immorality which leads to hell? I will try to explain by giving examples. Let’s say, someone is liar. He is a merchant, and lies about everything in such way that:
- He lies about his goods to customers, he sells them as high quality while they are the lowest quality. The customers might get hurt, and his reputation will will be bad, his business would stop.
- He lies to people in general, when they know truth, they won't trust him and probably have hate towards him.
- He lies about Allah and his messenger, saying that Allah and the Prophet said this is halal and this is haram while the Prophet and Allah didn’t. He can mislead people making them do sins, and he himself carries their sins as well as his.
From all what I mentioned, lying caused serious harm to others and destruction to the liar, it doesn't bring but harm and evil. Lying can cause many more serious problems if you think about it. That's why lying is a major sin. To prevent all the terrible consequences of lying, you have to prevent lying.
However, lying is permitted in only 3 situations as mentioned here:
عن أسماء بنت يزيد قالت : قال رسول الله صلى الله عليه وسلم : "يا أيها الناس ما يحملكم أن تتابعوا على الكذب كتتابع الفراش في النار ، الكذب كله على ابن آدم حرام إلا في ثلاث خصال : رجل كذب على امرأته ليرضيها ، ورجل كذب في الحرب فإن الحرب خدعة ، ورجل كذب بين مسلمين ليصلح بينهما." رواه الترمذي
So lying is permitted only for:
- Complimenting your wife to make her happy.
- Lying to the enemy during war, as war is a trick.
- Lying to two quarrelling people to make up between them.
I hope that answers your question.