Regarding why the firstborn receives a double portion of an inheritance:

Q. I would like to know why it is the firstborn son not the second or third born.

A. Thanks for your question. The bottom-line reason is because the Torah says so. Deut. 21:17 says, "The firstborn - son of the hated wife - he must recognize to give him a double portion of all that he has; since he (the firstborn) is the first of his (the father's) strength, the judgment of the birthright is his."

So the Torah tells us (a) that the firstborn son gets a double portion and (b) that the reason is because a firstborn is "first of his father's strength." Of course, that still leave the question of why the firstborn is the first of his father's strength.

The Abarbanel suggests that there's something inherently special about firsts. For example, we bring first fruits to the Temple, not just any fruits. Speaking as a parent, I can corroborate. While I love all my children equally, and each one is unique, having a first child is a completely different experience from having subsequent children.

But like I said, the ultimate reason is because this is what the Torah tells us. Any explanation is us human beings trying our best to understand.

Rabbi Jack's book Ask Rabbi Jack is available from Kodesh Press and on Amazon.com.