It matters not whether you are timid. People don't want to make fools out of themselves--and the surest way to do that is to preach an outdated religion. Back in the 1980s, it was reasonable--even though even then quite a few people didn't want anything to do with that religion. Even then, it would turn off more people than anything else. These days, it's even worse because anyone could research the religion in 15 minutes, and the scam is more likely to be exposed. It also shuts down whatever little opportunity you might chance to get in the world to better oneself.
And it's even worse if you are easily intimidated. Going to the door with such a message is a ticket to lost opportunities as it is. But, when you are uncomfortable approaching people at their own homes, doing so all day every day doesn't make it any better. That is like forcing someone that is deathly scared of snakes to handle snakes all day long. You dread getting up even more than normal (as is, it is a drudgery and wastes time, and causes one to lose social and business opportunities). Then, you get to the door--when a door gets slammed in your face, your fears are reinforced. Worse, when you reach a door where someone pushes a printout of proof that the religion is a scam right in your face, threatens you with a dog or gun, or calls the management of the apartment complex on you. Then, throw in a pious-sneer or hounder partner, and you are forced to get that householder to take the study.
Another bad thing is when conditions at a call make you sick. I saw one guy that came out, spitting blood into a mayonnaise jar at regular intervals, when I got dragged into that call. Plenty of people have gone to houses where roaches crawled into their circus bags or their purses. Or, rats and mice were running around houses. I myself have seen my share of filth, usually dirty dishes piled to the ceiling in the sink or dirty clothes piled up a meter or two high on the couch. Or, yards that are absolutely rubbish--bare dirt with nothing more than a chain link fence, or that has rubbish (often cigarette butts) that seems to be at least 10 centimeters deep. Other things I have seen were other men (always just plain men) trying to pick me up, partners getting mooched out of cigarette money in the guise of food money, men that have at least a dozen different prescription bottles on the table when I was dragged in, and people smoking non-stop while we were at the door.
Not to mention trying to defend the faith. When I was in, trying to get the person into the cancer was a priority. Usually, they wouldn't budge on their religion. The best were the Catholics that made it clear that they were going to die Catholic. I heard other denominations that would push their own beliefs, often as vehemently as I was supposed to be pushing mine, on me. Back then, we were supposed to make a good attempt to rebut their beliefs (now, the witlesses are much wussier and will run from such a discussion). And, if they had a question, you had to do the work and research the Washtowel publications to get answers before they got answers from other sources. (These days, when they can type it into a search engine, you have to already answer the question or run from the confrontation).