If you take guru, sadhu and sastra, it is sastra that offers the strongest bind of the three. If however one wants to go on for detailed issues of application (perhaps there is an argument for vidhi bhakti without a guru, but not for vaidhi bhakti) then there is definitely a requirement for a guru.
Kind of like two people can engage in a discussion about physics with a little bit of fundamental training but if one wants to get into aspects that revolve around application, formal training is required.
The question was not "Is a guru necessary?"
I do not at all doubt that a guru is necessary.
The question was about how to choose a particular guru - whether to go for Swami X or Swami Y or Swami Z (or Roshi A, Roshi B, or Father D or Reverend G?), and how to justify that choice.
Even within one tradition, the situation is everything but homogenous. Different teachers have at least slightly different teachings, different moods, different reputations, and different kinds of pressure exist in relation to each teacher.
I might personally not like a particular guru - but perhaps he has the most accurate knowledge of the Dharma, it's just that I do not know that, given that I don't particularly like him and don't spend much time reading/listening to him.
I have already experienced that talking about the same problem with different instructors can result in getting very different instructions.
Especially since the general point being made is that to a materially conditioned person (like myself), the mode of goodness is like sugar to a jaundiced person - unappealing. Does this mean that whatever I find unappealing, this is what I should take up? The more miserable I feel at doing something, the more I should do it?
How did you know which guru to choose?
Was it somehow never an issue for you?
Did you "just know" which guru is the right one for you, so there was no question choosing among them?
Or did you have such trust in all the initiating gurus that you were convinced that any one of them would be good enough, so you simply took initiation as soon as you fulfilled the formal requirements for it, with whomever was giving initation at the time?
(Or did you get initiated at the time when the "zonal acharya system" was in place, so you had no real choice in whom to take initiation from?)
And if it was a non-issue for you, could you tell some more about it - what were the qualities that you had, what were your circumstances, what certainties you had that in your estimation most significantly contributed to your choice of guru not being an issue for you?
Moreover, how were you sure, apparently from early on, that your understanding of the spiritual literature is correct, or at least satisfactory - so that you were confident to proceed on the path as you understood it to be - ?