Why would the SPD prefer a coalition where they are the junior partner to one where they are the largest party?

Question

In the 2023 Berlin elections, the result for the largest parties was:

Party	Seats
CDU	52
SPD	34
Greens	34
Left	22

(the SPD received 53 votes more than the Greens)

80 seats are needed for a majority, so this allows for majority coalitions:

Coalition	Seats	Mayor
CDU, SPD	86	CDU
SPD, Greens, Left	90	SPD
CDU, Greens	86	CDU

The Berlin news reports that the leader of the SPD is recommending her party to start coalition talks with the CDU. Such a coalition would mean that the SPD loses the position of mayor. In a coalition with Greens and Left (which is also the previous coalition), the SPD would retain the position of mayor. It would seem that it should be strongly preferable for the SPD to hold the position of mayor than to not hold the position of mayor.

Why would the SPD prefer a coalition where they lose the position of mayor to one where they can retain it?

Answer 1

After waiting for 24 hours, a few more details are now available. The Landesvorstand (state committee) of the SPD has now voted for taking up coalition talks with the CDU, and the Sondierungspapier (exploratory results paper) has been quoted in the media.

Two quotes make statements about why the current coalition of SPD, Greens and Left is appraised to be no longer a "durable and robust project" (dauerhaftes und belastbares Projekt):

Als Gründe führen die Sozialdemokraten „stark überwiegende Eigeninteressen der Grünen“ an. Diese hätten auch Ziele des bisherigen Koalitionsvertrages relativieren wollen...

Ebenfalls als Hinderungsgrund wird die „stark angespannte parteiinterne Situation bei der Linken“ angeführt. Die Sondierer schreiben: „Auf Landesebene bestehen erhebliche Zweifel an der Durchsetzungsfähigkeit verabredeter Positionen in der Breite der Partei.“

(Translation by DeepL)

As reasons, the Social Democrats cite "strongly predominant self-interests of the Greens". The Greens would also have wanted to relativize the goals of the previous coalition agreement...

The "very tense internal situation within the Left Party" is also cited as an obstacle. The explorers write: "At the state level, there are considerable doubts about the ability to implement agreed positions across the party."

I'll follow up with some speculations of my own about what you can read between the lines.

First, the SPD is divided between a left and a right wing. Mayor Franziska Giffey and parliamentary speaker Raed Saleh belong to the right wing, and currently hold a majority in the state party committee. But the left voices are strong. During the day, a number of them could be heard that strongly oppose a coalition with the CDU, among them the youth organisation Jusos, and (regionally) prominent party members like Mark Rackles, Sawsan Chebly or Annika Klose. At the last election for party chairwomanship, Giffey could only win 60% of the votes. (Here is a longer piece by a former SPD election campaigner outlining the left wing position.)

Continuing the SPD/Greens/Left coalition would mean to further strengthen the left wing agenda inside the party, with an inherent risk that when the candidates for the next state elections are selected, Giffey and Saleh will no longer be able to win the leader positions.

Second, the potshot at the internal situation of the Left reminds of the traditional problem the SPD has with this party: From their perspective, the Left is just another social democratic party that competes for the same voters. Strategically, the SPD dreams of removing them from the political landscape and returning to the comfortable situation of times gone by when they were the only credible left party in German parliaments.

Currently the federal Left party is divided and was barely able to retain its presence in the Bundestag. The Berlin state organisation is one of the few that are more or less undivided and has a track record of opposing both old-time GDR sympathisers and Querfront proponents like Sarah Wagenknecht. Their senators (government members) are visible and relatively popular.

Sending them to the opposition benches might look like a possible divide-and-conquer move from the viewpoint of the SPD.

Answer 2

If the CDU and Greens joined up they would be in the majority and the SPD would be left out. They have to weigh the risk of the CDU/Greens joining up instead of the Greens/Left being willing to join them. While SPD/Greens/Left would be better for the SPD it might be argued that CDU/Greens would be better for the greens as either way they won't be first and there is one less party to deal with in the majority.