Building as a single, marriage, and prenuptial agreement

No, the "assets" brought into the marriage remain untouched, but the increase in value is shared, including the increase generated from the assets brought into the marriage.

So if you bring stocks worth €100,000 into the marriage and they are worth €150,000 after 10 years, you would have to pay out €25,000 in case of divorce.

But then it would still have to be exactly the same shares (units) after 10 years. It gets tricky if you rebalance during that time. Is it still the same money or does it count as a new acquisition from that point on? Possibly your wife’s money allows you not to have to sell your shares to pay off the mortgage because they are currently at a low point. Everything is somehow double-edged. I find it especially difficult when there are children and one has to make sacrifices. The current loss of earnings might still be compensated with separate accounts, but how do you compensate for missing pension credits and for making sacrifices when climbing the next career steps? I am glad that we have similar spending habits and similar incomes and thus pool all money together and can fall back on the statutory regulations.

The thing with the stocks was just an example, although they have been lying around for a long time because I am more the dividend type, but with a house it's of course easier.
My wife will also have nothing to do with the follow-up financing of the house (remaining debt in 5 years is still 50,000€, so "peanuts").

She doesn't live extravagantly or have a consumer behavior that I don't like, I definitely want to mention that. She just financed an expensive car and is paying it off quite "brutally." Everything is okay.

Yes, it's all double-edged, that's why I find it really complicated.

A good family law lawyer can also provide advice. I once had an initial consultation with a lawyer, the bottom line is that you can exclude everything, but in the end, during the divorce, everything is basically renegotiated. This starts with the fact that the value of the brought-in assets can be assessed differently and then disputes arise over that. For example, it is also possible to opt for separation of property and offset any child care periods, etc., through more generous (than the statutory) regulations. So either more pension rights adjustment in the form of longer spousal support or also retirement provision in the form of transferring earned pension points during the years of child care. Going to a lawyer always hurts, but it is often advisable.

Prenuptial agreements make sense for clear issues, for example to save companies that might have to be sold in a divorce to create a balance. These are also things that every judge understands. But everything else, and especially the understandable attempt to secure one’s previous life achievements from the partner, becomes a pure gamble until the big day x. That’s why the approach is still to have a check done before committing and, if in doubt, to be ready to share success. In the end, it is a marriage, a joint team – period. If you can’t handle that and already think about how to get out unscathed beforehand, maybe it’s the wrong partner anyway. You can make prenuptial agreements as cool as you want, the legislator is your third partner in the marriage, i.e. +/- the entire Bundestag. And something about the laws is constantly changing. And those who marry for financial reasons won’t win in a divorce. That’s the wrong motivation. It’s cheaper to just not do it.

You paint a very negative picture of the concern or the motive behind wanting to conclude a marriage contract. It can also be about (and that is exactly what I see here with the OP) protecting the spouse in cases of unequal distribution of tasks, e.g., in matters of child-rearing and career opportunities. Or also protecting children from a first marriage, etc., etc. Your statement that one should consider it carefully remains undisputed, yet nowadays one simply does not know how the partner will be in 10, 20 or more years, or, with all due respect, how one will be oneself then. One has already witnessed personality changes in otherwise perfectly healthy people, apparently simply because they get older. Call it midlife crisis or dementia, one is never absolutely sure.