Usage of Phrases for 'I Love You'
The phrase used to say 'I Love You' in Spanish varies greatly depending on the level of love and the type of love you feel for the recipient. For example, friends or those who are dating, but not seriously may say 'te quiero.' This would literally mean 'I want you,' but is more like sharing an affection than being in love with someone. This phrase can be used between very close friends, to a pet, or to someone you have recently become romantically involved with but are not extremely serious with yet.
Those who are in love would use the term 'te amo.' You would never say this to someone you just recently met or have not been seeing for long. This is a more serious form of love. 'Te amo' is when you are truly in love with someone. It is a very strong phrase. If used too early in a relationship, for example, you might scare someone away or make them feel uneasy. You wouldn't say this to a friend.
The next level up is 'te adoro,' or more literally, 'I adore you.' Although in English this might not be stronger than what 'te amo' means, in Spanish this is a very strong phrase. You could use this phrase with someone you have a special love for, whether or not it be a boyfriend or girlfriend, or a spouse, but it can be used also to tell your own children. After all, 'adorar' means to adore, and aren't your children adorable?
The only stronger phrase in Spanish is 'te idolatro' which means 'I idolize you.' This phrase, although not incorrect, is rarely used and with reason. Idolizing someone is usually reserved for our imaginations and hearts. Be careful using this one, if at all.