Breakdown of Мне не нравится, когда ты жалеешь себя.
не
not
ты
you (singular)
мне
to me
нравиться
to please / to appeal
Questions & Answers about Мне не нравится, когда ты жалеешь себя.
Why is себя used here instead of тебя?
себя is a reflexive pronoun meaning "oneself." In Russian, whenever the person doing the action (the subject, ты) and the person receiving the action (the object) are the same, you must use себя. If you used тебя, it would sound incorrect, like "you pity you" as if talking about two completely different people.
What case is себя in here?
It is in the Accusative case because it is the direct object of the verb жалеешь (you pity). The spelling себя is used for both the Genitive and Accusative cases, but here it is the direct target of the action.
Why does the sentence start with мне instead of я?
To say "I like" in Russian, you use the verb нравиться (to be pleasing). This verb requires the person experiencing the feeling to be in the Dative case (мне). Literally, it translates to "To me is not pleasing..."
If мне isn't the subject, why is нравится in the "he/she/it" form?
The grammatical subject (the thing doing the pleasing) is the entire situation: когда ты жалеешь себя (when you pity yourself). Because an action or situation doesn't have a gender or plural form, the verb defaults to the third-person singular "it" form.