When it comes to link building one of the most important aspects of it is the optimisation of the actual link. Whether getting the link voluntarily or by contacting webmasters, you have to exert the utmost influence on the link itself. Unfortunately this is also a Catch 22.
When you start controlling the appearance of your link it stops being a natural link.
Thus Google can use filters on such a link to determine it’s ”manufactured”.
So there is a really volatile relationship between links being natural and useful for SEO. Most successful SEO techniques have been abused in the past by spammers so that Google tweaks its algo to curb overtly artificial link building.