When my son cry, we never rush to pick him up. When we hear him cry, we will respond immediately to let him know we are there, but we won't rush to give him attention. We will just ask him in a smoothing voice 'why baby? what's wrong?', sometimes he'll stop and we just continue talking to him in a soothing voice and let him play on his own. Sometimes he'll continue crying, then we'll go over and smile and him, and check if he needs anything (diaper change, uncomfy, milk, etc). Sometimes it still dont work, then we'll pat him or place our hand on his chest to comfort him. If still cry then we pick him up (but sit down, no moving, no rocking). So far there was only once that we needed to carry him for more than 10min to comfort him and it was when he had flu and couldn't breathe properly and wanted comfort.
Books always say you can never spoil a child younger than 6mths old but I beg to differ from my personal experience. Babies now are much smarter than before and they learn very quickly too. After my 3mths maternity leave, my maid took over the care of my son. She loves my son alot and won't bear to let him cry. So she picks him up everytime he cries. And he was only 3mths+ at that time, he already know who he can 'bully'. When I'm home and my maid is busy, he won't cry for us to carry him. But when my maid is in the room, he'll cry and cry until she picks him up. He knows that my maid will pick him up so he only cry when she's there! Even my maid says she regret giving him so much attention and she should have listened to us.
Me and my husband have been 'training' my son like that. Everything he cries, he won't get what he wants. He has to say it. When he still couldn't talk, we made him point out what he wanted, then slowly teach him how to name objects and make requests.
And now he knows that with us he cannot cry for anything he wants, he wants something, he learn to open his mouth and ask for it. "mummy, I want milk" or "daddy, I'm hungry. I want biscuit."
Crying just makes everyone more frustrated. He's frustrated that he can't get what he want, I'm frustrated cos i dont know what he want. So it's good to teach them from young, even when they can't talk, just keep repeating and one day they'll repeat after you