But if someone abandons the concept of nature and explains these things as 'just the way they are', how absurd it is to combine the assertion that the parts of the Whole are naturally subject to change with surprise or resentment as if this change was something contrary to nature - especially as the dissolution of each thing is into the elements of which it is composed. Dissolution is either a scattering of the component elements or the change of solid to earth and spirit to air, so that these too are subsumed into the Reason of the Whole, whether the Whole is periodically turned to fire or renews itself through eternal mutations.
And do not imagine that this solid and this spirit are the same as at original birth. All this was gathered only yesterday or the day before from the influx of food consumed and air breathed in. So what changes is the gathered influx, and not what your mother bore. Suppose now that this influx has close implication in your individual self: that, I think, has no bearing on the present argument.
I had to really read this passage a few times to try to understand what Marcus is saying here. In the first part of chapter 7 of Book 10, all he's really saying is he doesn't believe the the Universe was designed to be in a constant "detrimental" change state or to be condemned to simply "perish". Rather, he is alluding that the Universe has a greater end goal in mind - or at least it has determined that change and perishing are good for the health of the Universe (self-sustainment). And to further get at the heart of the matter: he is alluding there is a directing mind behind the Universe.
In the second part, he clearly thinks it is "absurd" that some people are so surprised at (and resent) events while simultaneously believing the Universe is random mix of events.
The last part - what I get out of it is: we as individuals, are a series of different people. We only ever live in the now. So the at-birth version of you is way different than the 40-year old version of yourself, which is different than the 50-year old version of yourself. All that matters is now.
(see also Citadel p. 149)