Hi, my not too scientific way of understanding it is something like this:

- Imagine a small point on the subject.
- This will reflect light in many directions, think of it as rays of light reflected from this small point.
- A bunch of these rays will travel more or less in parallel towards your lens.
- Now, to get the image sharp, the lens must bend those rays back into a small point on the sensor.
- When you use a small aperture, only the centermost rays are let through the aperture and it is "easy" to collect them in a single point on the sensor. Basically all lenses can do this.
- When you use a large aperture, there are more rays that must be bent correctly into this single point (a larger cross-section area of the lens is used), and to me it seems logical that this is more difficult to achieve.

When it comes to comparing sharpness at different apertures/focal lengths I find the interactive charts at slrgear.com quite useful. At least for the lenses I own, these graphs seem to reflect what I've also seen in real use. The lens you mentioned, 135L, is known for being very sharp even wide open. This is also illustrated here.