Legal permanent residents, also known as residents or green card holders, may apply to naturalize, or become a citizen after 5 years of being a permanent resident (or 3 years if received got your green card through a U.S. citizen spouse). You can file your application 90 days before your 5 (or 3) years are complete.
1. Immigration authorities can almost never deport you once you are a citizen.
Once you become a citizen of the United States, immigration authorities cannot deport you under any circumstances unless they denaturalize you. Immigration can only denaturalize you if you lied or committed fraud on your naturalization or green card application. This means that no matter what happens in your future, no matter what mistakes you make, you cannot be sent back to your home country unless your green card or naturalization applications were fraudulent.
2. Immigration authorities will be able to deport you from the United States if you are not a citizen, no matter how long you've been a green card holder or how long you've been in the United States.
If you are not a citizen, regardless of favorable considerations in your case, immigration will always have jurisdiction over you and the authority to deport you depending on your actions. And even if you do not commit any intentional crimes, accidents do happen. What if you are not a citizen, get distracted while driving, get into an accident, and kill someone? Depending on how you are charged criminally, there will always be a possibility you could be deported as a result of this accident. Neither extensive residency in the U.S. nor your good character before any accident or crime can guarantee you won't be deported.
3. You do not have to relinquish other citizenships to gain U.S. citizenship.
Contrary to some popular belief, the U.S. does not absolutely require you to relinquish all previous citizenship before naturalizing. While there are a very few exceptions and your case might be different if you hold a position of nobility in another country, the U.S. will let you retain your other citizenships when you naturalize.
4. You can travel outside the U.S. with much more freedom as a citizen than as a green card holder.
Many green card holders do not know that they do not have complete freedom to travel outside the United States with a green card. If you travel abroad for more than 6 months at a time, you could be denied re-entry when you come back to the United States. And this rule applies no matter how long you have been a resident. With U.S. citizenship, you are able to travel outside the United States without time restrictions.
5. You can apply for green cards for your family members much faster as a citizen than as a resident.
While the area of law governing whether you can give lawful status to your family is extremely complicated, in almost every case, your spouse, parent, or child can get a green card through you MUCH more quickly when you are a citizen instead of a resident. The process to apply for your family will be a year to several years shorter if you are a citizen. So if you're not naturalizing for yourself, do it for your family!
Overall there are very few reasons not to naturalize once you are eligible, so start the process now!