Why is it that every time someone does/says something positive toward me, I fear that they have ulterior motives?

I live with depression and when someone pays me a compliment, my mind won't readily accept it. One of depression's superpowers is to convince us that we're worthless by diminishing or even filtering out any positive evidence to the contrary. When evidence that we're not worthless (like a compliment) is presented, depression resolves this dissonance by asserting either the compliment is trivial or the person offering it must have some nefarious motive - or both. This game depression is playing in our mind is known by a cognitive therapy term called Diminishing the Positives. It's a type of thought distortion and it can become so automatic as to be difficult to interrupt and challenge. If you notice a tendency to think that positive moments in your life "don't count" because "I got lucky" or "they said that just to make me feel better", then you understand how this distortion applies to you. Challenging thought distortions with evidence of our worth is a powerful and effective skill against Diminishing the Positives. Now that you're aware of it, you can practice interrupting and challenging the habit of your mind to look for reasons why a compliment shouldn't be trusted or accepted.

I feel like this sometimes too. For me, it stems from having done the same thing to someone else (I was on the side of having ulterior/different motives, other than just complementing them, such as wanting them to be more open to me physically), but it can also happen when you've had that sort of thing happen in the past. If it happened before, I think it's not totally unreasonable that it would happen again, but then you won't be able to trust people.

When we have established learned behavior/habits they tend to stick with us for a very long time. We must reshape/recondition our thinking that not everyone is out to harm us.

You may have trust issues. Or just a general fear that everyone is going to end up hurting you in some way.

This is a consequence of low self esteem. You don't believe the compliment, so you think the person could be making fun of you, when that is almost always not the case.

This could be due to the fact something has happened in your past which makes you think everyone has an ulterior motive- of course, there are many other reasons, but having trust issues often springs from past events.

because you don't believe in yourself, which you should because you are capable of way more than what you are doing now

You may have seen someone or you yourself have gone through betrayal where another person has put on a kind exterior but they actually weren't kind at heart.

Trust issues are completely normal, everyone has them at some point, the problem is sometimes they can take over and get in the way of day to day life. The trick is to evaluate the situation, if you are talking to a really close friend or family think about how long you have known them and think about why they would want to do anything to upset you. If you don't know the person very well, then they are clearly not worth getting worried about.

I understand what you're saying, I've felt the same way. You feel this way because in the past, someone has used you through flattery/praise. As a result, you're suspicious of anyone who says nice things about you.