Chacham Ben Sion, in his Sefer Or L’Sion, Chochma u’Mussar (page 165), says that what really indicates that a person has Ahavat Yisrael is whether he is happy with his friend’s Simcha.
It is not enough just to feel his pain, but in his words, “the main thing is to be happy in his happiness.” That is the true test of Ahavat Yisrael.
He says one of the reasons that we are not happy with our friend’s happiness is jealousy. He discusses the concept brought down from the Yarot DeVash (others quote it from Rav Yisrael Salanter) that when it comes to the laws of Shabbat, or meat and dairy, and the like, a person will ask Rabbi and do what he says. But when it comes to a Din Torah, when a person has a complaint against his friend about money, if he loses the case, he walks out all upset. He might even yell at the judge. What happened? He’s willing to lose $1,000 on a cow that was deemed unkosher, but he won’t give his friend $100? The answer is that it hurts him that his friend is gaining. That is the nature of a person. Tzar Eyin/ It is difficult to see someone else get. But if a person has true love for his friend, that means that his friend is really like himself, and therefore he won’t feel bad at all. He’s happy when his friend gets. That is obviously a high level, but it is a test to see whether we still have what to work on or not.
Of course, Emuna has a part in this. The famous Ibn Ezra says that if a person really believed that he gets what he needs to get, and his friends get with he needs to get, it will never bother him.
He continues, saying that a person should strive to reach the level that he loves everyone equally, whether the man smiles at him or makes his life difficult. He says that in this way, we act likeHashem. Hashem gives life to people even when He is angry at them, even when a person is doing all kinds of sins. So too, we have to love our friends regardless of how we feel about them.
And if a person is still upset at his friend, he has not yet reached that level. Unless, he says, that this person has been deemed a Rasha, but that is a rarity. Therefore, in his words, the main thing in the mitzvah of loving your friend like you love yourself is that we have to treat everybody equally.
Your love for people should not come from anything else but loving them as a Jew, because all Jews are created equal.
Have a wonderful day.