The first reason that dating on the rebound is not usually a good idea is that when you find someone new, you have a tendency to try to replace your ex right away, rather than just find someone to spend time with. You could unwittingly be mistaking residual feelings about your ex for feelings toward a new person. Many times, a rebound relationship will go too far too quickly. You might find yourself in a serious relationship before you realize what is going on. A quick and immediate serious relationship right after the demise of a serious relationship is definitely not a good thing.
Another reason rebound relationships are an emotional hazard is because if you realize that you are not over your ex and want to end your new relationship, it can wreak havoc with the feelings of the new person you are dating. You have thus proceeded to, whether intentionally or not, break the heart of your new companion. Leading someone on when you are still hooked on or hurt from another relationship is definitely not a nice thing to do.
One more reason to take your time getting back into the dating scene is that most people need time to heal after a long term relationship ends. Moving on immediately can be emotionally unhealthy. It is always a good idea to immerse yourself in spending time with friends and family that care about you, but moving on to a different dating partner may not be the best thing. Besides spending time around people who care about you, get to know yourself as a single person. You may have been part of a couple for a long time. Get to know who you are as an individual again.
For these reasons, it is clear that dating on the rebound is not a bright idea. There are many things that can go wrong when you jump straight from one relationship immediately into another. Instead of rebounding into a relationship that may not be healthy, stop, take a deep breath, and spend some quality time with yourself. Think about when you think you will feel comfortable in a new relationship. Think about whether you even want another serious relationship in the near future or whether you want to spend time with many different people for awhile. Think about the kind of person you would want to be in a relationship with. Just take time out for you instead of getting trapped into the emotion hazard of rebounding.