One Piece's Monkey D Luffy has had many years to flesh out his personality. For starters, he's reliable, patient, practical, and stable. He can also be stubborn, uncompromising, and bullheaded. All of these traits are reflected by his Zodiac sign.
Luffy being born on May 5 makes him a Taurus. Taurus is a fixed sign. It's ruled by the planet Venus and its Element is Earth. Tauruses are also highly compatible with Scorpios and Cancers (like Nami). Here's everything there is to know about Taurus that can be associated with the captain of the Straw Hat Pirates.
As a fixed sign of Earth, Taurus is the least changeable of the signs. This can be taken to represent the unchanging attitude of Luffy. No matter what he's put through, he always maintains his childlike optimism. Taurus people are known for being practical and well-grounded. They also like to harvest the fruits of their labor. Likewise, Luffy, while something of a dreamer, goes about achieving his ultimate goal of finding the One Piece and becoming King of the Pirates in a realistic and pragmatic manner. However lofty his ideals and desires, he understands the hard work that must be put in to achieve them and puts in all of the required efforts to get what he wants out of life.
Since Taurus is a Spring zodiac sign, it's heavily associated with life. It speaks to one's physicality, eating habits, and general material existence. Sure enough, Luffy is brimming with vitality, has, to say the least, a healthy appetite, and enjoys a life based on things he can see and feel. Tauruses are also known to be stubborn but it's important to understand just what this stubbornness actually means. One way to interpret it is a sense of commitment; a Taurus will endure and stick to a project even if it takes years to finish. Likewise, Luffy's search for the One Piece, while long and arduous, is something he plans to see through to the end.
In a meta sense, this ideology can also be applied to all of One Piece. This series has been releasing new chapters for nearly 25 years now and it still has no clear end in sight. Oda is taking his sweet time and making sure his story is as well-told and fleshed out as possible. Going back to Luffy, his personality is further represented by Taurus' ruling planet, Venus. This planet represents the ability to love and find beauty in all aspects of life. They also know how to balance relationships with others.
Luffy's ability to "love" is questionable, but his enjoyment of life isn't. He can find fun and excitement wherever he goes. This is in spite of all the people who try to kill him and all the oppressed nations he ends up having to save. Even when experiencing the harshest adversity, he's able to keep wearing the biggest grin on his face. The most observable Taurus trait in Luffy is his sense of friendship. As a friend, the Taurus is loyal and always willing to lend a hand. If the connection is intimate enough, the Taurus will nurture it and do whatever they can to make sure it lasts. This can best be observed in the way Luffy treats his crewmates or Nakama. Together, they all share a bond tighter than most families.
There are a few more things that can be inferred from Luffy's zodiac sign, but nothing concrete. Regardless, Luffy has had plenty of opportunities throughout One Piece to prove himself a true Taurus. He'll probably continue to show off even more Taurus traits as the series continues on.
