Most Christians relate to Judaism more than to Islam, Hinduism, or Buddhism, or any other religion. So, my question is, what do Christianity and Judaism have in common so as to make them so related to one another? Thanks.
What Christianity and Judaism share is a part of salvation history, as presented in the Old Testament.
Salvation History (in German Heilsgeschichte) seeks to understand the personal redemptive activity of God within human history to effect his eternal saving intentions. This approach to history is found in parts of the Old Testament written around the sixth century BC, such as Deutero-Isaiah and some of the Psalms.
What Christians teach (in the main) is that the Bible is a record of salvation history and the relationship between God and Mankind. Jews and Christians agree that there is one God.
While the question is very broad, that in a nutshell is where common ground can be found.
The significant differences (books have been written) begin where Christians believe in how salvation history continues as presented in the New Testament. (Among the varied Christian sects the disagreements in detail run rampant). Those adhering to Judaism disagree, and follow a different understanding of the continuing relationship between God and Mankind since they do not hold Jesus as having been the promised Messsiah.
Per your request for amplification on 'the relationship:' the relationship between God and mankind is established in Genesis and continues on throughout the Old Testament. It follows the pattern of God and mankind being close, then mankind turning away (the Fall) and then a series of turns towards God and turns away from God, among God's people, as time goes on. C.S. Lewis wrote of that pattern in his summary of the Bible. Further examples of the relationship are found in the covenants made with Abraham, Moses, and David.