You are obviously a "clean" user. By that I mean that, possibly, you do not use the internet extensively or, when you do so, you stay on known, and clean, sites?
Bear in mind that a huge percentage of enthusiastic users are youngsters who often download pirated material or visit sites which are linked to them.
Unfortunately, their chances of picking up a virus or trojan, without protection, are of a very high risk.
I must confess that, throughout my computer days (going back to the ZX80) I also did not think an antivirus program very important. Fact is, I did not experience a virus attack until well into the XP era, when, on advice, I began to protect my computer.
If you browse and read up on it, you will find that today you cannot guarantee your safety even on reputed sites.
If you are happy to merely have an anti virus program installed but not resident, you can run it say, weekly, and hopefully you will survive. There is not a great deal of overhead in having the program running from start up and I would adise it.
If, on the other hand ( this is my own method) you ever are aware that you have been attacked, have an image or some kind of squeeky clean backup handy and bung it back on.