The verb אהב (love) is always linked with את. And when it is written ואהבת לרעך כמוך (you shall love your neighbour as yourself)⁷²³, it means he would love for his friend to attain whatever he wants to attain for himself. And this is the difference between the word את, which teaches about the essence of the action and the letter ל, which teaches about the attainment of the matter.

It is impossible for a person to love his neighbor as he loves himself. However, it is possible for a person to love for his neighbor to attain whatever he wants to attain for himself. This is why the Torah uses the letter ל in Leviticus 19:18 when commanding a person to love his neighbor as himself⁷²⁴.

footnotes: ⁷²³ Leviticus 19:18. ⁷²⁴ Malbim, HaTorah v’HaMitzvah Kedoshim 45.