Answer by taking a quote from the following article:

Clemens was born in Dayton, Ohio, the fifth child of Bill and Bess (Lee) Clemens. He is of German descent, his great-grandfather Joseph Clemens having immigrated in the 1880s. Clemens's parents separated when he was an infant. His mother soon married Woody Booher, whom Clemens considers his father.

Clemens unexpectedly appeared in the owner's box at Yankee Stadium on May 6, 2007, during the seventh-inning stretch of a game against the Seattle Mariners, and made a brief statement: "Thank y'all. Well they came and got me out of Texas, and uhh, I can tell you it's a privilege to be back. I'll be talkin' to y'all soon." It was simultaneously announced that Clemens had rejoined the Yankees roster, agreeing to a pro-rated one-year deal worth $28,000,022, or about $4.7 million per month. Over the contract life, he would make $18.7 million. This equated to just over $1 million per start that season.  Clemens made his 2007 return on June 9, defeating the Pittsburgh Pirates by pitching six innings with seven strikeouts and 3 runs allowed. On June 21, with a single in the 5th inning against the Colorado Rockies, Clemens became the oldest New York Yankee to record a hit (44 years, 321 days). On June 24, Clemens pitched an inning in relief against the San Francisco Giants. It had been 22 years and 341 days since his previous regular-season relief appearance, the longest such gap in major league history.  On July 2, Clemens collected his 350th win against the Minnesota Twins at Yankee Stadium, giving up just two hits and one run over eight innings. Clemens is one of only three pitchers to pitch his entire career in the live-ball era and reach 350 wins. The other two are Warren Spahn (whose catcher for his 350th win was Joe Torre, Clemens's manager for his 350th), and Greg Maddux, who earned his 350th win in 2008. His final regular season appearance was a start against the Red Sox at Fenway Park, in which he allowed 2 hits and 1 unearned run in 6 innings, and received a no-decision. Clemens finished the 2007 regular season with a record of 6-6 and a 4.18 ERA.  Clemens was forced to leave Game 3 of the 2007 ALDS in the third inning after aggravating a hamstring injury. He struck out Victor Martinez of the Cleveland Indians with his final pitch, and was replaced by right-hander Phil Hughes. Yankees manager Joe Torre removed Clemens from the roster due to his injury, and replaced him with left-hander Ron Villone.

Why did he return to the Yankess?
Clemens unexpectedly appeared in the owner's box at Yankee Stadium on May 6, 2007,