It is proper for a person to improve himself because of himself, apart from Allah, he also tells us to improve ourselves. Not because of other people, not because of expecting a good partner. Because a good partner, will automatically follow, if we have tried our best to improve.
The problem is, something done because of someone else, or done because you expect something else (because you expect a good partner, for example), will only invite a lot of disappointment. On the other hand, something done for the sake of Allah, because of a good understanding in itself, will always be reassuring. No matter how difficult the process is.
And what must be understood, because whether it is a lifelong process, not all good women or men are lucky to be met with their respective partners in an equally good condition. There are some who are not less fortunate, who are united in conditions where one of them is not very good, but has the potential and the desire to always improve. It remains a matter of time and patience, what is not good will equal or even exceed what is good.