Why did the Social Democratic Party of Germany enter a grand coalition with the CDU-CSU in 2005?

The Social Democratic Party of Germany (SPD) entered a grand coalition with the Christian Democratic Union (CDU) and its Bavarian sister party, the Christian Social Union (CSU), in 2005 because the federal election held that year resulted in a virtual dead heat between the two major parties.1

Following the election, neither the SPD nor the CDU-CSU bloc received a clear mandate to govern, and negotiations with smaller parties failed to produce a viable governing majority. Consequently, the parties formed a "grand coalition" to ensure the country had a functioning government, with Angela Merkel of the CDU becoming chancellor.123