The L'Dofkei B'Tshuva cites the Beit Yitzchak (B"Y) who states that the act of Teshuva needs no specific accompanying intent to fulfill the mitzva - unlike other mitzvot. Teshuva is all about internal, personal regret; thus, any prerequisite that demands that upon repenting one must have in mind that God commands so is inimical to Teshuva. This is why no blessing is recited before performing T'shuva. To state that 'God commands us to repent' before repenting defeats the purpose. Repentance must be intrinsic - it loses all meaning if there is a requirement to think about/state via blessing how God's teshuva command is THE reason that I changed.
He compares this to a commoner who sins before a king. He or she would not dare say to the king that he or she is now sorry because the king demands this. Such a statement is highly insulting. Rather he or she will confess his or her profound, personal sense of remorse and wrongdoing.
Relatedly, the (M"C) Minchat Chinuch (Mitzva 364) posits that per the Rambam there is NO obligation to do teshuva. (If you want to do it and do so per the Torah's ways, that teshuva is a mitzva - just like the mitzva of divorce):
רק כיון דתשובה אינה מצוה כמ"ש אם כן מ"ע זו הוי כהרבה מצות מהתרי"ג שכ' הר"מ בסמ"ק שאינה מצוה לעשות או שלא לעשות רק העשיה על תואר כך וכך נחשב מצוה כמו גירושין
What happens to the Beit Yitzchak's no-requirement-to-do-teshuva-for-God's-sake upon analyzing the M"C?
A sins. A does not per the Torah need to repent. But if A chooses to do so properly (at which point his actions are actions of 'commandment'), shall we say that there is no requirement to intend to do this act due to the 'commandment' because this would undermine the intrinsic and personal nature of repentance? This would be the normative B"Y approach.
Or would the B"Y say that per the M"C, a demand is placed on A (a la all the other mitzvot) that A think about the teshuva 'commandment' as the impetus for A's actions given that there is no actual obligation to repent and thus such a I-did-it-for-God-once-I-chose-to-opt-for-it kavana would not undermine the truly volitional nature of teshuva?
If the latter approach is true and that the B"Y agrees that per the M"C a prerequisite kavana to A's teshuva exists - this became a 'commandment' because A chose to change his or her life and thus A must think of fulfilling this 'commandment' - the B"Y would likely posit that per the M"C there must be another reason no blessing is made before repenting.