This is an excerpt from Edmund Burke's "Of the Effects of Tragedy" from his book On the Sublime and Beautiful. Can someone please explain what he is saying in this paragraph, especially the first sentence? I believe that this notion of our having a simple pain in the reality, yet a delight in the representation, arises from hence, that we do not sufficiently distinguish what we would by no means chuse to do, from what we should be eager enough to see if it was once done. We delight in seeing things, which so far from doing, our heartiest wishes would be to see redressed. This noble capital, the pride of England and of Europe, I believe no man is so strangely wicked as to desire to see destroyed by a conflagration or an earthquake, though he should be removed himself to the greatest distance from the danger. But suppose such a fatal accident to have happened, what numbers from all parts would croud to behold the ruins, and amongst them many who would have been content never to have seen London in its glory? Nor is it either in real or fictitious distresses, our immunity from them which produces our delight; in my own mind I can discover nothing like it.

Your question refers to an excerpt pulled from the fifteenth chapter of part one of a much larger philosophical treatise. It's worth noting that this fifteenth chapter, "Of the Effects of Tragedy," is itself closely intertwined with its preceding chapter, entitled "Of the Effects of Sympathy in the Distresses of Others." In fact, Burke himself begins this fifteenth chapter with the words, "It is thus," referring to patterns he'd already outlined in that earlier chapter. With that in mind, to recognize what Burke is arguing in this chapter, it's useful to be aware of what he argues in that earlier one.
The fourteenth chapter is ultimately focused around the claim that human beings actually tend to feel some quality of pleasure in the suffering of others. (Indeed, before going any further, it should also be noted that Burke tends to focus a lot on pleasure and pain: these are, for him, the cornerstones of human emotion.) That being said, this pleasure is not actually sadistic—in fact, for Burke, it's a constructive quality, because the alternative to this is, for Burke, a state of general apathy. In Burke's own words, "the delight we have in such things hinders us from shunning scenes of misery," which in turn gives opportunity for sympathy and empathy to do their work.
From here, we come to the fifteenth chapter, which builds upon this theme. For Burke, as we already pointed out, suffering draws peoples' interest. Note, at the end of this section of the passage you've provided, the example Burke provides: by which London is devastated by fire or earthquake. No one wants to see the city burn down, just as no one actually takes great pleasure in the suffering of others should they directly witness it, but as soon as these sorts of incidents have happened, that tends to draw interest. This is what Burke means when he writes, "we do not sufficiently distinguish what we would by no means choose to do, from what we should be eager to see if it was once done." While people don't wish these kinds of tragedies and disasters on other people, once those tragedies have happened, they tend to draw interest. The experience of real suffering, compared to what one might perhaps label a representation of suffering (whether it be tragedy in its dramatic sense or whether it be instead an awareness of real-life tragedy which is occurring at a distance): for Burke, I'd suggest that there is a qualitative difference between the two in terms of how people tend to respond emotionally to them.