The harder to get the link, the more valuable it is

Links that are too easy or relatively easy to get do not help much in getting traffic or authority for search engine rankings.
If your link is placed on a page where there are several hundred links competing for attention, it is less likely that potential visitors will click than if the page only has a few dozen links.
The value of your link source is in direct relation to how selective that source is when placing links on the page and how much traffic the source gets.  The value also declines with the number of links on the page.
Google is understood to use algorithms to measure the importance and quality of each page.  The PageRank was invented by Google founders and is used for measuring absolute importance of a page.  The TrustRank algorithm describes a technique for identifying trustworthy pages — quality pages.  We can not tell for sure to what extent Google is using this algorithm if at all, or at least their publicly known version.  What we can say, is that based on observation, we can definitely say that they do not treat all links equal and they do not pass authority to your page from all of your link sources.