1. The main difference was religion and ethnicity. If you were a Jew, a Roma person, a Slav, or anything other than an "aryan German" then you would probably end up in a concentration camp which almost meant certain death. The differences were superficial and in practice did not even exist but were perceived as great.
2. Not necessarily but often they do. To be in a conflict means to be in collision with someone else on the topic of something. These can be solved through words or diplomacy or an agreement, but often they result in conflicts such as fights or even wars. The conflict that was world war 2 was a great tragedy that was extremely difficult to avert since Hitler never wanted to end the conflict.
3. The holocaust shows us that we are all people and that the monstrosities committed by the Nazi regime were not just crimes against a certain population but against the entire human race. The tragedy shows us how superficial and untrue it all was and how wicked the regime was and what happens when you let the worst parts of you outside.
4. People stick to what is similar to them which was seen in Germany and their people and their army while any perceived difference is seen as dangerous. Germans supported Hitler because he represented them but what they couldn't understand was that we are all human above everything and that there are similarities between them and the Jewish population too, not just differences. We need to remember this so as to not make a mistake like that ever again and we need to keep in mind what brings us together and not what pulls us apart.
5. There are various reasons to forget such things and most are bad. One can be that they want to let bygones be bygones and keep moving forward while some might consider it to be too big of a trauma that should just be suppressed and forgotten. Others do it for political reasons because they want fascist or neonazi parties to get power again so they twist history to suit them.
The reasons to remember are plenty as well. We need to remember so as to not repeat the history for starters. It is also necessary to remember those poor people since forgetting them would be the same as being complicit in the horror. People need to learn from that even and know how to fight evil at every step and not just live in ignorance.
