And you shall love your fellow Jew like yourself
Vayikra (Leviticus) 19:18
The Torah commands us to love our fellow Jew like we love ourselves. This is one of the most important mitzvot, because many commandments depend on it. For example, if one loves his fellow Jew then he would not steal from him, or hurt him in any other way.
A non Jew once came to the great sage Hillel saying, "I wish to convert on the condition that you teach me
the entire Torah "while I stand on one foot" - "in a very short time"." Hillel accepted him as a convert and
then told him, "What is hateful to you do not do to others, this is the entire Torah, everything else is
commentary, go and learn. Talmud, Shabbat 31a
In addition to the mitzvah to love one's fellow Jew, we are also commanded not to hate another Jew. This is one of the most serious sins.
You shall not hate your brother in your heart
Vayikra (Leviticus) 19:17
One reason why this is such a serious sin is because it can lead to the violation of many other prohibitions, from lashon hara (slanderous speech) to murder. Another reason why hating another Jew is such a serious sin is because G-d loves every Jew. How then can we hate a person that G-d loves?
Why was the first Temple destroyed? Because of three sins, idolatry, adultery, and murder... But the second Temple - when the Jews were
involved in Torah study, mitzvot, and doing kindness - why was the Temple destroyed? Because of the baseless hatred which existed at that
time. This teaches us that baseless hatred is considered equal to the three sins of idolatry, adultery, and murder. Talmud, Yoma 9b
Acts of love and kindness are so much a part of Jewish Law that the word "mitzvah" (commandment) is commonly used to mean any good deed. Jewish Law includes within it a blueprint for a just and ethical society, where no one takes from another or harms another or takes advantage of another, but everyone gives to one another and helps one another and protects one another.
The Ten Commandments command us not to murder. Jewish Law requires us to protect our fellow man. We are commanded not to leave a condition that may cause harm, to construct our homes in ways that will prevent people from being harmed, and to help a person whose life is in danger.
Jewish Law commands us to help those in need, both in physical need and financial need. The Torah commands us to help a neighbor with his burden, and help load or unload his beast, to give money to the poor and needy, and not to turn them away empty handed.
Jewish Law forbids us from cheating another or taking advantage of another. Jewish Law regarding business ethics and practices is extensive. The Torah regulates conduct between a businessman and his customer. For example, not to use false weights and measures, not to do wrong in buying and selling, not to charge interest. The Torah also regulates conduct between a business man and his employee. For example, to pay wages promptly, to allow a worker in the field to eat the produce he is harvesting, and not to take produce other than what you can eat from the employer while harvesting.
Jewish Law forbids us from wronging another person in speech. The Torah commands us not to tell lies about a person, nor even uncomplimentary things that are true. To speak the truth, to fulfill our promises, and not to deceive others.