There are three sorts of abstinence, actually: the kind that the Torah itself requires of us (when it charges us to avoid this or that); the kind that the sages added onto those, known as “safeguards”, to make sure we don’t come close to doing something the Torah itself forbade; and the kind that those who are striving for piety would take upon themselves to ensure they don’t even veer toward wrongdoing. We’ll be focusing on the latter sort, for while the first two are incumbent on all of us, the last is optional and are the very sort that the pious would choose.
Now, a question could reasonably be asked here. As Ramchal himself words it, “What right has anyone to add on prohibitions” beyond the ones the Torah and sages instituted? In fact, as he points out, the sages themselves raised the question when they said, “Hasn’t the Torah forbidden enough things for you — would you forbid even more?” (Jerusalem Talmud, Nedarim 9:1). After all, he goes on to offer as devil’s advocate, “what our sages saw fit in their wisdom to prohibit and to use as a safeguard should certainly be so, but what they saw fit to allow should be allowed and not prohibited. So why would anyone come up with decrees that the sages didn’t see fit to make?”
Then he adds (rather tongue-in-cheek), “There’d be no end to this” sort of thing in theory. “It would come out that a person would have to suffer (all sorts of) desolation and deprivation, and derive no pleasure from this world at all!” if this kept up. Why, “haven’t our sages said, ‘You’ll have to give a reckoning and account before G-d for each permitted and available thing your eyes saw to eat that you didn’t eat'” (Jerusalem Talmud, Kiddushin 4:12). So how could anyone justifiably take new restrictions on himself in light of all that?
“The only response to that”, he then offers, would be the following, In point of fact, “abstinence is certainly necessary. Our sages (Sifra) exhorted us about it by saying that when the Torah says ‘Be holy’ (Leviticus 19:2) it means to say ‘Be abstainers’. They also said that ‘If the Nazirite (who would abstain from drinking grape products, from cutting his or her hair, and from attending funerals) is called ‘holy’, we could extrapolate from there that anyone who fasts is all the more so to be called holy” (Ta’anit 11a), which implies that abstaining from food (or the like) as much as possible is seemingly laudable. And he goes on to offer other traditional statements which all “clearly indicate the need for and the obligation to practice abstinence”.
The solution to this seeming contradiction is simply this, as he words it: “there are the abstentions that we’re commanded to follow” — each one of us — such as the ones the Torah and Tradition laid out. And then “there are abstentions that we’re cautioned to observe so as not to come to stumble” but not commanded to, which those who are striving for piety would eagerly take upon themselves because it suits their loftier goals. The point is that the rest of us needn’t take those restrictions upon ourselves in fact, though we might draw lessons from them and reconsider our ways in light of them.