Rav Chaim Greinman was one of a kind. He was one of the gedolei hador, yet outside of the yeshiva world nobody ever heard of him [why this is so is not important for our purposes]. He was a nephew and close talmid of the Chazon Ish and unfortunately passed away 3 years ago. He wrote amazing chiddushim on all of Shas from Brachos until Uktzin - something few people in history have done. He was also an expert on medical matters and helped many people in many different ways. In addition to being a Gaon - he was a Tzadik.
When he was sitting shiva for his brother, he remarked that when he was forbidden from learning Torah, he would think about gehenom. He was asked why he doesn't think about Gan Eden?? He answered - "Why should I think of others when I can think of myself".
[עי' ברכות כ"ח: - וכשחלה רבי יוחנן בן זכאי, נכנסו תלמידיו לבקרו. כיון שראה אותם התחיל לבכות. אמרו לו תלמידיו: נר ישראל, עמוד הימיני, פטיש החזק, מפני מה אתה בוכה? אמר להם: אילו לפני מלך בשר ודם היו מוליכין אותי, שהיום כאן ומחר בקבר, שאם כועס עלי - אין כעסו כעס עולם, ואם אוסרני - אין איסורו איסור עולם, ואם ממיתני - אין מיתתו מיתת עולם, ואני יכול לפייסו בדברים ולשחדו בממון - אף על פי כן הייתי בוכה; ועכשיו שמוליכים אותי לפני מלך מלכי המלכים הקדוש ברוך הוא, שהוא חי וקיים לעולם ולעולמי עולמים, שאם כועס עלי - כעסו כעס עולם, ואם אוסרני - איסורו איסור עולם, ואם ממיתני - מיתתו מיתת עולם, ואיני יכול לפייסו בדברים ולא לשחדו בממון; ולא עוד, אלא שיש לפני שני דרכים, אחת של גן עדן ואחת של גיהנם, ואיני יודע באיזו מוליכים אותי - ולא אבכה?]
If people would believe in gehenom, they would act very differently. It is usually the Tzadikim, who do no aveiros, who are the ones who are most concerned with gehenom. The people who are really going there are completely at ease. Things are so coooooolllll!
I was sent a link of an article about a "religious" doctor. He davens, wears tallis and tefillin, keeps Shabbos, kosher etc. etc. He says that he believes in it all. What does he do for a living? He castrates people!!! Indeed - he tries to turn men into women and women into men. All day, every day. Why? Here he has a disagreement with G-d. He thinks that it is the right thing to do even though he knows that G-d forbade it. It helps that he is very well paid. Money has a way of shaping ideologies.
What about gehenom? If he has no moral problem with doing what he does - what about his ETERNITY???? I mean even if it is even 50-50 or even 10-90 that he is going to regret his actions FOREVER - wouldn't he stop? If he was told that performing such operations causes extremely painful brain cancer in 5 percent of the doctors - wouldn't he find other ways to make a living? Probably. So what about the fact that his own tradition which he believes in tells him 100 percent that he will regret his actions forever and ever??
For that he needs yiras ha-onesh. But that is what he lacks. I am guessing that he does some other aveiros as well. If one doesn't have any fear - he can do what ever he wants. What is going to stop him? He keeps Shabbos - it is a nice cultural tradition. Kosher - makes him feel Jewish. But the moment their is a clash between he desires [for example - to earn money] and the Torah - the Torah must fall by the wayside רח"ל.
The basic and most fundamental feeling to prevent sin is fear. Ideally - יראת הרוממות - awe and reverence of Hashem's presence. On a lower level - יראת העונש, old fashioned fear of punishment.
We all need it - in addition to the higher feeling of love of Hashem which should guide all of our actions.