The reason for their 'crummy backgrounds' is to illustrate how each character dealt with the adversary of their lives. By far Gaara had the worst background. While his mother was pregnant Gaara's father used a jutsu to combine the unborn Gaara with the sand demon Shukaku. His mother then died giving birth and cursing the life of her son to be filled with everlasting pain and loneliness. Then for the first 6 years of his life everybody feared and hated openly. And then by his father's order the only person Gaara thought he could trust and loved his tried to assassinate him. Gaara killed him(his uncle) but before he died he told Gaara that noone had ever loved him and infact hated him. After this for the next 6 years his father ordered numerous assassinations of him. It doesn't get any more screwed up than that.