Some people have said "I've been putting my hands on people or animals since I was a kid (or for years)
and my hands heat up and I heal them, so I don't need an attunement, I'm already doing "Reiki".
How does a Reiki Master Teacher respond to a comment like that? Number one, that may be so, but you are not channeling Reiki, no how, no way! Unless you have been connected to the energy via a ceremony called an "attunement" by a Reiki Master Teacher, you cannot channel Reiki pure and simple.
Number two, more than likely you are using your own personal energy to "heal" others and THAT IS NEVER A GOOD IDEA! Using your own personal energy to heal can cause serious illness and possible death. Here is how it woks, when you are born you come in with X amount of "natal chi", the energy you need to fully function in your body for as long as you are alive in that body, which is stored in your kidneys (according to Chinese Medicine). Think of it as a pie. When you have difficulties in your life like major illnesses, loss, major stress, etc., visualize slices of your "pie" being used up. When all your slices have been used up, you leave the body and transition.
You can replace some of the pie slices (cultivate more chi, Prana) by doing things like T'ai Chi or QiQong, and some yoga breathing exercises, all of which are designed to bring more "chi" (life force energy) into your body, also getting adequate and deep restorative sleep nightly, eating a good healthy diet, drinking plenty of clean, fresh water and getting out in a nature regularly help to increase your chi. So using your personal energy to heal others uses up your pie slices faster!
You might subscribe to a belief system or practice that teaches you how to connect to and channel the energy of the earth, heaven, fire, air,etc. and are using this when you put your hands on people for the purpose of healing. Again, this is not Reiki, and you may run the risk of tapping into your own personal energy again along with these other energies.
Other people claim to have sat in meditation and "pulled down" a Reiki attunement. I have a say as a Reiki Master Teacher and just as a skeptical human being in general, I don't think so! A Reiki attunement optimally is performed by a certified and trained Reiki Master Teacher in person, one-on-one with another human being. It is not something you can pull out of the air, just doesn't work that way, sorry.
I don't pretend to have all the answers. I am just telling my truth as it was taught to me by my teachers, my personally experience and feelings, and what I have read and understood in a slew of Reiki books and articles I have read in over a decade.