Break ups can be rough. They can, well, break you. People need ways to cope after going through something so hard. Some people find their healing in the place of someone else. More specifically, in the place of a new relationship.
Dating someone right after a breakup is not the way to move on and heal. It will only hurt. After dating someone for awhile, it can be easy to forget life being single. The person who you are always used to being right there suddenly isn't. No more date nights or phone calls.
So, running into someone else's arms won't teach you how to adjust and be independent. The time after a breakup is the time to be selfish and just take care of yourself. It's when you're supposed to call your friends and rant endlessly then watch sad movies and eat ice cream. After the moping period, that's when you pick yourself up and find enjoyable hobbies and simply embrace the single life.
If you date someone immediately after, it won't allow you to fully process the breakup and move on. It takes time to move on from a person you were so close to. Like grieving a loss, there is a process. You need to focus on your mental health and make sure you are completely over that person before involving yourself in someone else.
The next person you date should know the best version of you, not the hung up version. You should date, not to get over a breakup, but because you genuinely enjoy the person. So give it at least four months. In those four months, healing should take place, not hookups and relationships. After four months, or however long it takes to move on, then date someone and hopefully it will be a better relationship than the last.