[Lord Elrond]

I think since Tolkien on only had two sons of Earendil and Elwing one of them had to choose mortality and the other elf kindred so Tolkien could continue with the continuity of his story. I don't think there is any in depth reason why however if I were to make believe I will say that he was a daddy's boy and wanted to be just like his father who was mortal. He might have even had the delusion that he could become immortal like his father (presumably immortal) through his own accomplishments. However I think its just due to the fact Tolkien chose Elrond to be Immortal so the other brother had to be mortal.