Lenses are usually high tech pieces of technology that are made out of a lot of different types of components; optic-related components, electrical components, mechanical components, etc. The photography sector is nowadays a very big sector and firms have to produce 'better' (more zoom, fast autofocus, image stabilizer, big aperture, etc.) lenses to maintain a good position in the business.
BUT, at the same time, those lenses usually do not have to be very big, heavy and (probably mainly) expensive, at least not for the majority of the targeted audience; normal people with a photography hobby.
Lenses usually are already a compromise between different aspects like durability, image quality, price, weight, etc. But due to the constraints (lenses have to be light, small, cheap) some parts of the lens cannot be built with the ‘ideal’ specifications: strong, durable, quality feeling, etc.; the strength of certain components could be less than ideal, the connections (with glue, welding, screws, whatever) between parts could be less strong or durable, the internal wear could become an issue after time, etc.
So, lenses in general are already quite fragile instruments, compared to furniture and cutlery for example. When a lens is used intensively the wear increases and besides that, after a period of time the chance of breaking or jamming just increases. Abusing lenses/ focus mechanisms (turning when it is not possible, very wild turning) will make the probability of failure higher. Dropping a lens (or letting the front end of the lens hit other objects) is for the same reason quite dangerous.
On internet forums and youtube for example there are a lot of individuals who give tutorials about fixing specific issues; tightening screws inside a lens are an example of one of those issues(tamron 17-50) ,but tutorials of fixing broken autofocus mechanisms are there as well. After searching a bit on the internet I have found some opinions about repairing like:
I don't think anyone without the proper training and tools have ever attempted to repair a lens and ended up with anything other than a pile of useless parts. Either replace it or have it professionally repaired.
Anything is possible, but I doubt it's wise.
(Both from forums.popphoto.com)
The safest way would be to send it to Canon Service Centre or a repair shop and ask for a quote on the repairs.
So at least you have to be attentive when you want to fix a lens yourself, there is a reason why repairing at 'pro centers' is often expensive.
But still there are enough people who fix the problems without doing even more damage to the lens and have a lot of fun/profit of it. Of course is not every lens problem the same, but when you face an issue it seems smart to me to look on the internet if it is an often occurring problem with a known treatment. When you have decided to do it on your own you have to work well structured; in a clean, well lit room and you have to know exactly every step you take, because you have to do all those steps 'in reverse' after the fix.
Tl;dr: A lens (and so is the focus mechanism) is often made out of a lot of relatively fragile, small parts which could break quite easily. When you want to fix it on your own you have to do research and you have to have a certain amount of knowledge and skill. There are never guarantees.