The speculation regarding the motives of those (not just Nicola Sturgeon - remember 2014 ?) wanting Scotland to become independent can be approached from many different angles. One such is to simply view other examples with open eyes, totally fearless of the bias often presented and preached so fervently.
There is a wealth of statistics and general information on the relative performance of different countries. Time and again the same collection of fewer than 10 countries come out as the best performers when it comes to a wide range of socio-economic parameters. Then the question is what is it that makes these countries so good at these things ? They are all of a similar population size to or smaller than Scotland. Most of them have no gas or oil resources and with the exception of Norway (who pile up the money for the day it runs out, refusing to spend it now) all who do have less of the stuff than Scotland - in any case, even before the 70s North Sea oil exploration/exploitation they were doing enviably well. Again and again the UK trails well behind.
The argument that Scotland is subsidised in net terms by England is something that is repeatedly brought up, as is the suggestion that an independent Scotland would possibly/probably not gain membership of the EU. This, until it is tried and tested, is no more than pessimism and/or ill will, perhaps both and more negativity besides.
For the negative predictions, any of them, to come to pass for an independent Scotland, the people of Scotland would have to be incompetent by comparison to said ten or so countries. In fact, all that an independent Scotland would need to do would be to literally copy the administrative/management/etc. systems in use (in some/most cases for the past decades) in those countries. Personally, I have no doubt at all that Scotland could come up to compare pretty well with the standards of well being enjoyed in the Nordic countries, the Netherlands, Switzerland, etc. My only reservation/fear is that nostalgia, inertia and hesitation would seriously slow down (hopefully not hinder altogether) the escape from the backward UK state of affairs because even though the numbers might change to more than 50% in favour there would still be substantial numbers of people who would not wish to let go of the dead-weight UK model. Additionally, the politicians might lack the very substantial imagination, initiative and courage to push on with the changes needed. When applying for EU membership an independent Scotland should present itself realistically - a backward breakaway from a backward country needing a good bit of support until it reaches its potential. I strongly disagree with those who will insist that Scots are inherently inferior (to for example the English).