How long have you been working in testing? And for how many companies/customers/teams?
What you are speaking about is indeed a very common issue. It has a lot to do with overall “team dynamics” and the psychology of testing.
Our job is to question what other people did (or are going to do) in order to increase the overall quality of a product. Some people see us as partners. Other people see us as dangers, who will question their work. Some people just think we are useless because they feel that the product they are building does not need improvement (and this last type of people is really hard to work with!). Other testers will probably have other types of colleagues to add to the list.
During my carrer I worked with all types of developers, from people who were so enthusiastic about my work that it was freaky, to people who were downright hostile every time I tried to work with them. It is okay. Promoting our work is part of our job. Many developers have never worked with a tester before, or worse, they have worked with “bad” testers. Testers who would despise them and make fun of their mistakes. Testers who were former poorly skilled developers and were moved to a QA position in a bet from HR to “make something useful” of them. Testers who would not do their job properly, and give a GO to products that were simply not ready.
You need to earn the confidence of this developer, for whatever reason. It can be done directly, by showing your worth, or by proxy, by involving another person who is enthusiastic about your work (you know, the fanboy who asks you for help every time he makes a commit?). In order to achieve this, what I found useful is to have a chat with another developer who has a great work relationship with the developer I was trying to “reach”. Do not be inquisitive, just ask how he/she sees the job, what that person thinks about quality, check if he/she may have had bad experiences with testers before… And so on.
Persist! If you do earn the trust of this developer, in the future, he/she might become your best ally. And you will probably learn a lot of useful things in the process.
I wish you good luck.