One would say that it is just for those to whom e-liquid mixing is a hobby and, it can be true in most of the cases nevertheless, there are also other motivators.
Money might be evaluated as the second strongest motivator, since mixing by yourself should be favorable in terms of expenses. So, it should be that in the end you get the desired e-juices at a better cost. Of course, in the beginning mixing by yourself can be more expensive than buying one or a couple of e-liquid bottles from the store, but as one gains experience in the mixing process and gets all the needed ingredients in his/her own warehouse, the expenses per bottle decrease significantly.
Transparency of ingredients that e-liquids contain is another strong motivator for those who have decided to mix by themselves: in the reality, unfortunately, one can’t be sure about what ingredients are used in branded e-juices. It is due to the fact that e-liquid manufacturing companies don’t specify in details what they put in. Therefore, for now the person who wants to know 100% about all ingredients used in e-liquid he/she is smoking has no other option than just start mixing by him-/ herself.