Many students get hung up on writing thesis statements before they have written their essays. This can lead to writer's block, and writer's block can lead to frustration and anxiety. I suggest that you forget about writing a thesis statement until you have written your paper. Just because the thesis statement comes first doesn't mean that it has to be written first.
You have three obvious parts to your essay and should probably deal with them separately, focusing on one at a time. First comes how Hamlet and Laertes are similar. Then comes how they are different. And finally comes the question of why they are similar and different. I suggest that you try to avoid comparing and contrasting at the same time. After you have written a first draft of your paper, you will be in a better position to think about your thesis statement, which would be based on what you have said in the paper. Have you thought about how Hamlet and Laertes are alike? Have you thought about how they are different?
The biggest "why" question, it seems to me, is why Hamlet doesn't act as impetuously as Laertes to the death of his father. This is because they are two different types of young men, which should be explained in the section of your paper describing how they are different.
I hope this answer has helped. The best advice I can think of is to forget about your thesis statement until you have at least written a first draft of your essay. Your problem with a thesis statement is one of the most commonly asked about problems received by eNotes. The problem, in my opinion, is usually caused by people trying to say what they are going to say before they have thought about what it is they are going to say. It's hard to know what you are going to say until you have said it.