After September’s election, the SPD ruled out working with the CDU, prompting the latter to enter discussions with the liberal FPD and the Greens. These talks soon collapsed, and the SPD agreed to discuss options with the CDU.
The country is still months away from having a fully-functioning government, but the chances of a renewed alliance between the country’s two main parties have just grown.
As of last year’s elections, the Bundestag is the most diverse in the post-war years. At the vote, the far-right AfD came third and the SPD and the CDU each lost votes and seats. A renewed alliance runs the risk of weakening the country’s centre-ground and strengthening its extremes.
One can only presume the SPD hoped to wade it out as a strong opposition while a fractured, three-way, ideologically diverse Merkel-led government fought amongst itself, so a likely return to government will not help them carve out their own image in opposition to the centre-right's.