When Yosef brings his two sons, Ephraim and Menashe, to his father Yaakov, to bless them, we find Yaakov does something quite curious:

שכל את ידיו – he maneuvered his hands… (48:14)

He switched around his right hand with his left, so that Ephraim, the younger son, was under his right hand. But why did he not just tell Menashe and Ephraim to switch places?

R’ Chaim Volozhin explains that the nature of the average person is to talk down the positive attributes of his fellow, but when it comes to the negative side, he is ready to speak up and even exaggerate the other’s bad traits. In this way, he will look good compared with his friend.

Indeed, when one stands opposite his fellow, one’s left is the other’s right and vice-versa. This hints to the fact that his friend’s right, or stature, stands opposite his left, generally the one seen as weaker, i.e. weakening his friend’s strengths, whereas his friend’s left, i.e. weaker side, is opposite his right, i.e. contrasting his own strengths with his friends weaknesses.

Yaakov was the ultimate man of truth, not wishing to detract or embellish any characteristic. He only crosses his hands – the result being that his right hand corresponds with the boys’ right and his left with theirs!

It teaches us a really important, says R’ Moshe Shternbuch – how careful we must be when relating to the strengths and weaknesses of our friends (and others!), placing our right opposite their right…

He cites the Rambam for further proof – ”it’s a mitzva upon every person to to love his fellow Jew as himself as it says ואהבת לרעך כמוך. Therefore, he must tell of his praiseworthy points…as himself…”.

As an addendum, Gav C pointed out that the Ohr Hachaim says a beautiful idea associated with the above: it says שכל את ידיו כי מנשה הבכור – he maneuvered his hands because Menashe was the firstborn – surely Menashe being the firstborn would be reason to not cross hands!?
So he answers that just before, we were told that ”Yisrael’s eyes were heavy from old age and he could not see”. This indicates that he couldn’t see who was Menashe – the firstborn – and who was Ephraim. So when it came to blessing them, he knew who he wanted under his right hand. He worked out in his mind that Yosef would have placed Menashe on his right – therefore he crossed his hands because he knew where the firstborn was, and because Menashe was the בכור!