not sure why the subjunctive is used

Scis quam iniquus interdum, quam impotens saepe, quam quelior semper sit amor. I supose the sentence injects a certain amount of doubt and thus the use of sit but other than that I am not sure.

It’s indirect question construction.
“How unfair is love?” or “How unfair love is!” — indicative, est
“You know how unfair love is” or “Do you know how unfair love is?” — subjunctive, sit

quelior??

The original:

C. PLINIVS PAVLINO SVO S.

Irascor, nec liquet mihi an debeam, sed irascor. scis, quam sit amor iniquus interdum, impotens saepe, μικραίτιος semper. haec tamen causa magna est; nescio an iusta, sed ego, tamquam non minus iusta quam magna sit, grauiter irascor, quod a te tam diu litterae nullae. exorare me potes uno modo, si nunc saltem plurimas et longissimas miseris. haec mihi sola excusatio uera, ceterae falsae uidebuntur. non sum auditurus ‘non eram Romae’ vel ‘occupatior eram’; illud enim nec di sinant, ut ‘infirmior’. ipse ad uillam partim studiis partim desidia fruor, quorum utrumque ex otio nascitur. uale.

Pliny, Letters, 2.2.

μικραίτιος - querulus? petulant? in context: quick to get angry over trivialities.

A reminder to catch up on my lagging correspondence.

I guess μικραιτιος was a word in vogue, and it is hard to think of a good Latin equivalent.

Anyone want to try translating Pliny’s letter?

You can translate μικραίτιος as “prompt to resort to petty accusations over trivial matters.”

Why not?

Pliny sends greetings to his Paulinus.

I am angry, and it is not clear that I ought to be, but I am. You know how unkind love sometimes is, how it is often uncontrollable, and always > nerveux> . But this is a great cause for anger; and I don’t know if it is a just one, but as if it were no less just than it is great, I am deeply angry, because no letter has come from you for so long. You can appease me in one way: if now, at least, you send many letters of great length. This is the only apology I will accept; others will seem false. I shall not listen to your “I was not at Rome” or “I was very busy”; and as for “I was sick”, may the gods not allow it to be so. At my villa I am partly engaged in study, partly in idleness, both of which arise from leisure. Be well.

I know it’s not perfectly literal.