When did this become tradition? To give up something for lent? All my Roman Catholic friends do it. But it seems to me that after the Church stopped fasting together with the Black Fast, this became normative, and led to a individualization of the fast. Is this supposition true?
Jesus answered them, "Can the wedding guests mourn as long as the bridegroom is with them? The days will come when the bridegroom is taken away from them, and then they will fast. 11
11  Fasting is a sign of mourning and would be as inappropriate at this time of joy, when Jesus is proclaiming the kingdom, as it would be at a marriage feast. Yet the saying looks forward to the time when Jesus will no longer be with the disciples visibly, the time of Matthew’s church. Then they will fast: see Didache 8:1.
There are actually few days during the 40 days of Lent when fasting from three meals of food is required.
Abstaining from meat on Friday accounts for 6 days.
I’m imagining, that in order to get a deeper experience throughout all 40 days of Lent, that the religious and laity developed a daily observance - and gave up something throughout all days of Lent.
The best resource is jimmyakin.org. Log on to that web site and look for the “Lent Fight Update” posted on March 8, 2011. It has links to all things about Lent.