Time does not heal all wounds. Ahem, Snape.
Keep your friends by you. Harry would not have survived year one, let alone the rest of his adventures without loyal friends.
Where your treasure is, there will your heart be also. This is probably the most prominent theme of the entire series, stated in book seven. Will Harry treasure power, fame, or will it be loving others no matter the cost and seeking to overcome evil?
The capacity to love more than the extent of one's abilities determine the capacity for greatness, as shown by Neville Longbottom.