What is the meaning of [Care killed a cat]

Care is the load on the mind. If it killed a cat, which has nine lives – or so
’tis said – it will kill _you_ , who have only one life. So, don’t worry – get
the load off your mind. George Wither, the Jacobean poet, wrote of Christmas:

> Hang sorrow! Care will kill a cat,
> And therefore let’s be berry.