<human>: Ok so sorry if any of this is a bit dumb, i'm a relative noob when it comes to personal finance. 

A bit of background... when i was 18 i moved onto a student account with HSBC with a £1000 overdraft. At the same time they offered me a £500 credit card, which initially i didn't want (my dad had about £50,000 in credit card debt and it ruined him... i didn't want to put myself in that situation of temptation), but the lady walking me through creating my account said it would be a good idea to take it, use it every once in a while and pay it off straight away to help improve my credit score. By the end of University i had basically spent all that. 3k a year in student loans wasn't really enough to live off so i would spend the overdraft (interest free at the time) and the credit card was for emergencies. i'd then work part time over the summer to pay off the overdraft so i'd have it next year. so after university i graduated with about £1500 of debt (credit card paid for getting to interviews) 

Now i'm 23 and been working full time for over a year earning about £1600 per month after tax NI and Student loan repayments etc. and have been slowly paying off my credit card and am about £500 away from paying off my overdraft. 

Over the weekend my bank sent me a letter out of the blue saying they would be increasing my credit limit to £1500 (an increase of £1000). i can decline it if i ring them up... but I didn't ask for it in the first place and i know banks don't really do this kind of thing for "no reason" so i was wondering if anyone had any insight as to either why the are doing this, or if there is something i should be cautious of in terms of it affecting my credit score or anything like that?

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: Is there any reason i should be worried about my bank automatically increasing the limit on my credit card form £500 to £1500 next month? and also why would they do this without me asking?