This little poem comes from Disticha de Educatione of Urbano Appendini, published in 1834; you can see the whole book at Google Books.
Te gravibus veluti in rebus nimis esse iocosum,
In laetis nolo sic nimis esse gravem.
The vocabulary is keyed to the DCC Latin Vocabulary list. There is only one word in this poem that is not on the DCC list:
iocōsus -a -um - humorous, joking, jesting
gravis -e: heavy
in: in, on (+ abl.); into onto (+ acc)
laetus -a -um: glad, joyful
nimius -a -um: too much, excessive; nimis or nimium, excessively
nōlo nōlle, nōluī: be unwilling
rēs reī f.: thing (rēs pūblica, commonwealth; rēs familiāris, family property, estate; rēs mīlitāris, art of war; rēs novae, revolution)
sīc: in this manner, thus; sīc . . . ut, in the same way as
sum, esse, fuī: be, exist
tū tuī tibi tē: you (sing.)
velut: even as, just as