Why do emails which I send out get thrown into a recipient's spam file?

I have sent out emails to people who later told me they found it in their spam file. I know it is less likely to happen if you are added to their address book. However, What is the typical criteria an email provider uses to determine an email to be spam?Why do emails which I send out get thrown into a recipient's spam file?
Certain words trigger all spam filters unless the filter is set to not trigger. One of the words is viagra. Another word is spam. If the email says "business proposal" this is also likely to trigger it. Whatever is commonly found in spam will get your message trashed.