Is it permissible to supplicate for the restoration of hair density that was lost after it had been thick, and is grieving over this considered a lack of contentment with Allah's decree?

Excessiveness in supplication is to overstep its bounds by asking for something that is not permissible in Islamic law, such as sin or severing ties of kinship, or something that contradicts God's cosmic laws. What happened to you, the reversal of hair due to illness or allergy, is not excessiveness; rather, a person's supplication for healing and for averting harm is commanded by Islamic law. There is no harm in supplicating to God to restore your hair to its previous state. The heart's sadness over hair loss is a natural human emotion and does not necessarily imply dissatisfaction with God's decree. The Prophet, peace and blessings be upon him, said: "Indeed, the eye sheds tears, and the heart grieves, but we do not say except what pleases our Lord."