In Romeo and Juliet, why is Capulet holding a feast in the first place?
Shakespeare doesn't actually give Capulet a specific motive for holding the feast. It's just, I suppose, a party that you'd throw because people like having parties - it's also, if you think Capulet's party is particularly lavish and expensive (no real evidence for this though!), sometimes done in productions as being a statement of Capulet's wealth and standing.
The only other motive you might argue for it is in this speech, in Act 1, Scene 2 of the play. Capulet's talking to Paris - and this is the first mention of his feast that we get:
But woo her, gentle Paris, get her heart;
My will to her consent is but a part.
An she agree, within her scope of choice
Lies my consent and fair according voice.
This night I hold an old accustom'd feast,
Whereto I have invited many a guest,
Such as I love; and you among the store,
One more, most welcome, makes my number more.
At my poor house look to behold this night
Earth-treading stars that make dark heaven light.
Such comfort as do lusty young men feel
When well-apparell'd April on the heel
Of limping Winter treads, even such delight
Among fresh female buds shall you this night
Inherit at my house. Hear all, all see,
And like her most whose merit most shall be...
Might Capulet be holding the feast specifically to get Juliet and Paris together? Perhaps. But then why won't he let Paris marry her for two more years? In short, I'm not sure there is a defined motive for Capulet's feast!