To be able to convince others, being able to take no for an answer is a good starter. If your boss feels like he only can say yes, otherwise you will have bad feelings, he will feel pressured and say no more likely.
By being able to take a no in good stride, you make him feel free to decide what's best.
Being open to feedback is a good thing, but expect that most product feedback turns into a no. It's much easier to have ideas, than to turn them into reality.
There might be a myriad of technical and non technical reasons to say no:
- Political reasons within the company: Maybe that technology/solution is burned, and it would need political captial to have another go at it. Political capital your teamlead might not even have, and if he has it, he might spend it otherwise
- legal reasons. I had a coworker once propose a Machine Learning solution, which would have been much better. But that didn't matter, the most important thing was that we could explain to customers and possibly a judge why our system decided what it decided. With ML, the quality would have improved, but the explainability would have suffered.
- the hidden assumption: When 2 or 3 technical proposals collide, and everybody is sure their is best, their might be a hidden assumption in there. Maybe about user behaviour, user/customer expecations, usage, data shape, or something else. Everybody might be right if THEIR assumption is the correct one. The trick is to make that assumption visible, and then find out which one applies. E.g.: I worked several years at a company where downtime could be measured in money. We always knew how much each minute downtime costs. (This depend on weekday, time, weather, month, and a few other factors. But we always knew). So uptime/reliabilty was a huge factor. When I came into the next company/project, I brought that assumption with me. I had disagreements with people, until I found out: Downtimes don't matter, people just have to wait. If the whole software doesn't work for one weekend, that's fine. Just fix it monday morning. Of course, that changed the solutions I pushed for once I realised that.
- Cost/benefit analysis: You might be right, your solution might be better. But is it worth the cost/effort? Somethings might only be worth it at a certain scale, if the customer base is big enough, etc... Also factor in cost of maintenance, not only development: If you use something fancy/exotic, it usually is more effort for the Operations team to keep it in production. They might need to hire somebody with specialised skills. Also, what if the development team has to grow? if you use many fancy solutions, it will be thougher to findsome who knows all that. Or more expensive to train them up instead
- other reasons still
So I would first advice to accept that it is perfectly fine to accept feedback, and then not do what is proposed. Doing so gracefully is a skill, if your PO is lacking in that, help him develop it. If he needs to explain better, ask for better explanations/context. Be open to hear him out.
Shooting down most suggestions and wanting more suggestions can be perfectly consistent: You want to hear everything, accept the one suggestion that's a goldmine, and decline the rest. That includes stuff that is merely good.
Another very general point: Even if you are right, and the company should do what you propose, how you go about it matters. It's though to sum up all advice on good communication in a stackexchange answer: There is a reason there are whole books about this topic. As programmers, we often feel the best solution should win, how one argues for it shouldn't matter. I understand that idealised vision, but the how often matters more then the what. So if you take courses on communication, or read books, or watch videos that will benefit you in the long run a lot.
What you can do in the short run:
Talk to your PO, in a meeting with only you two, when you have enough time and quiet to talk this through. Explain to him that you feel sad that all your proposals didn't get accepted. When doing so, only state facts and your personal feelings. Speculations about his motivations or feelings aren't facts. A fact could be: I proposed something 3 times. You said "No, we won't do this" to all 3 of them. Only use his verbatim words.
Then ask to help you get your ideas accepted: What can you do so he accepts them? You want to bring your ideas/creativity to the team, so ask him to help you do it. If he is serious about wanting feedback/suggestions, he will be open to discuss this.
Hopefully, you will be able to find a solution that's acceptable to the both of you.