Very pleased with that. Germany always needs a strong SPD.
True. They actually gained a decent percentage, admittedly from a very weak position, but still. The German system is very different to the UK, so makes no sense to compare Labour and German Social Democrats either (edit: I know this wasn't you). Germany is used to coalitions, the only one party majority government we had was in the early Sixties, on a nationwide scale basically no one could ever govern without a smaller partner (or as grand coalition).
In addition to the difference in popularity between SDPs leader and Merkel's successor, don't forget that the SDP have also been a part of the administration so they also benefit from the fact that people are pretty happy with things and that a vote for them isn't a great departure from the status quo or too big a risk
The funny thing is that for the past 16 years with Merkel as chancellor the SPD spent 12 years in the grand coalition and kept on haemorrhaging votes. Regardless of how okay things went, and they actually did quite a bit of good in those years (minimum wage would not have happend without them for example). But many people credited CDU/CSU and Merkel with the successes and the overall stability. Drove me nuts in the last two elections. At least now some saw the light of day, just in time you might say.
I cannot look into the minds of my fellow countryman, and you can bet there are enough idiots here as elsewhere, but from what I would guess (and understand from friends and family) the result is a combination of people being tired of a conservative led government (they have been 'in power' for 16 years damn it) and the fact that Armin Laschet turned out to be the wrong candidate. I would not go so far as to call him an idiot like stoa - there are definitely worse politicians in his party - but he really was hapless and never came across as someone you could see on the big international stage, he neither projected sufficient confidence nor competence. And yeah, we want to forget about the whole clusterfuck leading up to him becoming candidate, too.
Plus generally the political landscape has changed and has - just as in other European countries - become more fragmented and former smaller parties like the Greens have gained ground while the "big two" (SPD and CDU/CSU) have lost voters, especially among the younger population. Or rather they never had these; first time voters in their majority are not exactly conservative voters especially these days with various crisis on all fronts (climate, social, immigration etc.). Someone mentioned slow digitalization, although this probably played a smaller part. It is a problem that exists mainly in rural areas, in most mid/larger towns and the big cities there is decent to good digital infrastructure. So if the small entrepreneur living on a farm in the middle of nowhere has trouble getting his files to business partners did not vote conservative due to not having an internet/mobile connection, this will not have made a big overall impact. Unhappy as some might be with their digital situation (and it sure has an impact on them), they do not form a majority, or at least it will rarely be the main reason for a vote.
Thankfully the AFD has lost a bit, although I still see a double digit figure and it leaves me speechless. How anyone can vote for this fascist and racist neo Nazi party - in Germany of all places - is just something which is beyond me and I think it is shameful. Admittedly it is mainly in the 'new' Eastern states like Saxony were they got majorities and in central areas and cities they have single digits only (it is something like 7% where I am in Frankfurt/Main, which is still seven percent too much if you ask me).
Anyway, there will be some interesting weeks ahead. It will all hinge on whether the Greens and the Liberals can find common ground. Basically out of all the four main parties they are the furthest apart in some key aspects, so this will prove tricky. But both sides have already signalled they will talk with each other and try to sort out something. Whoever they then pick - the social democratic or the conservative offer - is actually the second step. Of course officially they will be picked, but for now they are in the driver's seat. I really hope they will get their shit together (although I could have done without the liberals) and we will get what has been dubbed 'traffic light'. It is rather preposterous of the conservative party to still claim they want to govern with Laschet as chancellor, when a) they only came in second, b) they have lost almost 9% while the Social Democrats have gained 6% and c) barely anyone wants Laschet as chancellor. If I were him I'd be glad to be able to say no to the job and they should just go and be the main opposition party for the next four years. How hard can it be, it is democracy after all. Not a conservative trait to admit defeat though. Really ridiculous.