[getting better]

You can lay this down ONLY when you can lay this down without asking. If there is some doubt then you should still call. There is rarely no doubt unless there is more than one other player and there has been some action leading up to your decision.

Also, even if this is someone who you figure wont bluff here, consider that he could know you have the straight and still be raising the same hand as yours in hopes of winning the other half of the pot.

And finally, he is not certain that you have the straight.