Stop wanting to feel entitled to anything from anyone,
    or stop thinking that anyone can show themselves grateful.
Everything brings no return; it is no use to do kindness --
    indeed, actually, it is tiring and more: harmful.
At least it seems so to me; whom noone vexes more sharply or heavily
    than he who now has me as his one and only friend.
Desine de quoquam quicquam bene velle mereri
    aut aliquem fieri posse putare pium
Omnia sunt ingrata, nihil fecisse benigne,
    immo etiam taedet, (taedet) obestque magis*
ut mihi, quem nemo gravius nec acerbius urget,
    quam modo qui me unum atque unicum amicum habuit

It seems in this poem that Catullus is sort of pissed off at the world, mainly for the reason cited in the last line, that someone either just won't leave him alone, or has committed some sort of harm to Catullus. He's also, based on the Latin words which he uses, railing against the Roman social system (which was based on networks of mutual loyalty and gratitude). Its ambiguous enough, however, that the translation I've provided ought to have no claim to the true meaning, it just happens to be what I think makes the most sense. The meter is elegiac couplet.

* In the original version of this poem, this line is unmetrical: "immo etiam taedet obestque magisque magis". Most later additions of the poem follow an addition first made by Avantius, which adds a verb to close out the previous line's last clause and makes this line "prodest, immo etiam taedet obestque magis". But, imho (and in that of some commentaries), this makes less sense than what I'm using here, which keeps "it is no use to do kindness" by itself. The taedet in parentheses is added to fit the meter, and the alternate suggestion of Avantius, so the line should literally translate to "... it is tiring, tiring and more harmful".

Latin text from:
Aronson, Andrew C. and Robert Broughner. Catullus and Horace. White Plains, NY: Longman, 1988
Some commentary from:
Fordyce, C. J. Catullus. New York: Oxford University Press, 1961.