I've never once asked for a significant other's password to their phone or any account they may have online. I know so many people who get upset if they don't know the password to their significant other's profiles or phone, and I really don't know why.
When asking for someone's password or login, I see it as a violation of trust. Why do you need to know their login? What do you need to look for? Are you searching for something? What my significant other does with their phone is their business. I don't need to know about every conversation they have with someone, and it really isn't any of my business. It belongs to them.
I trust my significant other enough to know that they aren't doing something they shouldn't be behind my back. if I didn't feel that way, then I would wonder why I am in the relationship in the first place. Relationships are built on trust, and if that isn't there, I shouldn't be in one.
See also: When It Comes To Social Media Accounts In Relationships, What Is More Important Privacy Or Sharing?
What am I going to benefit from if I have their phone password? I can do what I need to on my phone or computer. The only way I may need to know the password to their phone is if I need to respond to someone while they're driving. Regardless, if it were that important, it would be a phone call, and I wouldn't need a password to answer it.
If my significant other asked for my passwords, however, I wouldn't think twice about it. They can have any password they want because I've got nothing to hide. They aren't going to find anything on my phone or messages that they don't already know about.
Afterward, I would probably be a little skeptical. I know in the past I have wondered, "Why did they ask? Do they not trust me? Do they think I'm hiding something?" I don't want my significant other to be in that situation. I don't want them to think I am trying to find something or like I don't trust them; it isn't fair to them.
Like I said before if you need to have your significant other's password for whatever reason, you probably don't trust them. There is literally no reason why you need it, and you shouldn't feel like you do. Give them their privacy because they deserve it.
You never know. They could be planning something special for you, and if you snoop, you may ruin the surprise.