I can't remember which episode it was, but there was one where Odo says that he's suspicious about anyone who's willing to help a police officer. After all these years I still don't understand that line. Why would Odo say something like that? Why WOULD it be suspicious when somebody wants to help a cop? You'd think that Odo of all people would appreciate it, since that's exactly what he is...
So, this is my speculation but I cannot remember the full context so I might have missed something. Odo had primarily been working with the Cardassians, and Bajorans under Cardassian rule. As such everyone has an ulterior motive other than law and order. They either want something from the officer, they want something against the person they are helping the officer against or they are seeking favor. He also is dealing with Quark who always has a secondary motive.
Not just willing, but eager. This dialog was from the episode "Rules of Engagement," when they were trying to figure out why the Klingon civilian transport decloaked in front of the Defiant in a combat zone. The people Odo was questioning were eager to talk to him, but none of them could or would give an answer to the one question that mattered. They were probably feigning cooperation as a tactic to mislead or divert. There are words for people who are eager to help with police investigations, such as "canary," "stool pigeon," "snitch," etc. They are likely somehow involved in the matter themselves and are trying to generally throw off the investigation, or even get a particular person in trouble. Anything they say should be taken with a dose of skepticism. Let's face it. Cops make us nervous. If a police officer is questioning us about anything, we have our guard up more than usual. We don't fall all over ourselves to divulge more than exactly what they ask, unless maybe we tend to talk too much when we're nervous. But we're certainly not eager to answer their questions. Kor
In general, even people with a positive attitude toward law enforcement will always have the suspicion in their heads that any cop they meet is looking for something to accuse them of.
Let’s also not forget that in his early career Odo was the only policeman who even cared about justice and even he made a mistake that cost innocent people their lives.
A cop would at least keep in mind the possibility that the helpful person's motivation is not what's on the surface. Framing someone they don't like? Deflecting his suspicions away from them or their friend?
Odo is a very honest police officer, but he's also jaded. He doesn't even fully believe in the law. He says laws change with governments, and he's had experience as a police officer under a despotic government with no respect for sentient rights. OTOH, he thinks the Federation's laws make it harder for him to catch criminals. "Untie my hands!" he says. He says he knows who the troublemakers are and wants unlimited powers to ban them public areas and search their person and effects. He keeps track of who's sending an abnormal volume of messages, even people who are not under suspicion. He doesn't see himself as a community police officer, a member of the community whom people have granted limited powers within the framework of the law. He sees himself as separate, a benevolent bringer of justice, possibly an attitude related to his Founder origin. So when someone seeks him out, he starts with the premise they're up to something, for the reasons @Kor and @fireproof78 mention.
Indeed, Odo probably grossly misinterprets his inner fire as a desire for justice, when in fact it is a desire for order. He wants everybody to keep on doing their ordained thing in an orderly fashion, since deviating means they are trying to upset the system, and the system exists solely to protect the Founders. People lining up to help the police in crowd control or hunting down the cattle rustler are probably fine, at most currying for favors. People willing to help the police in their inquiries are up to no good, because volunteering information is inhuman, and volunteering pertinent information suggests involvement, interest and intrigue. Statistically, Odo no doubt is correct in his assessment. Also, he recognizes prejudice for statistical fact when appropriate (without prejudice towards this act of recognizing)... In other words, he's gruff. Which is both a tool of trade for him, and second - naah, first - nature. Leaving this thing unsaid in the "Rules of Engagement" context would have done no service to anybody. Timo Saloniemi
Because they're masking that they're hiding something. Too eager = dodgy. Too reticent = dodgy. An innocent person fits somewhere in the middle.
Seriously?? I like Odo's statement from "Emissary" -- "I don't allow weapons on the promenade. That includes phasers."
Even police don't respect a rat, especially one eagerly coming forward. Its one thing to fold under questioning and another to volunteer information, the latter shows a weakness of character, lack of loyalty and potential to manipulate events for personal gain. All qualities any intelligent person would be wary of when dealing with that individual .