Elias confesses eventually that the reason he wants to make Chise his bride is because it is convenient. Let's take a moment to process that.
Beware the spoilers, and let's get right into it.
Ancient Magus' Bride (Mahō Tsukai no Yome) has two main characters: Elias and Chise. Early on, Elias admits that aside from training Chise as an apprentice in magic, he would also like to marry her. Chise was probably more surprised than anyone else since the title of the series spoils that moment. But it still felt abrupt and strange, not to mention questionable, seeing how young Chise is. And as we know how old Elias is, the questionable nature of this decision is further highlighted.
Later, however, Chise begins to understand that Elias is not that well-versed in emotions. He does have emotions, which seem to be rather strongly connected to Chise, He feels "cold" without her and "warm" with her, and he does seem to develop rather strong emotion whenever Chise is in danger. However, that just further highlights that his intention to marry Chise was a little too early, seeing how back then, he did not know her, did not feel for her, and was not even sure she would live for more than 3 years at that time.
The 3 year deadline was revised by Chise's encounters with more and more dangerous creatures, resulting in Chise having much more time. And the two do marry after Chise's birthday, at least, in their own manner. After all that, though, Elias states that he thought having an apprentice and wife all in one would be convenient, which is why he did just that.
That is a whole new take on marriage of convenience. Rather than having relationships with two separate people, Elias decided to have different relationships with one person. As if that never caused trouble or resulted in both relationships being ruined due to some incompatibilities.
To be fair to Elias, he said it in response to Chise asking why he wanted a marriage when he did not fully understand what marriage meant. He seemed to have an idea that to perpetuate a sorcerer's line, a marriage was needed, but he did not seem to know much more. In other words, Chise, once again, just might be the more aware and educated here, just like with emotions.
Elias is a strange creature in the world of Ancient Magus' Bride; he is neither fae nor human, and his origins are not clear. Despite being so much older than Chise, he is still learning to be around a human so much, and that puts them on the same level, at least to some extent. Chise has things to learn from Elias and things to teach to Elias. As a result, their interactions are even more satisfying (and less creepy) as they both explore things they had not known existed.